I'll just chime in with some useful pieces of advice first some fundamentals

[list]
[*]Forum roleplaying is typically turn based
[*]You and your opponent has a turn to make actions. Actions in roleplaying combat is extremely important.
[*]In each turn you may attack, defend, or evade attacks your opponent proposes and your opponent is rewarded the same
[*]A hit can only be confirmed when an attack cannot be blocked, evaded, or mitigated in any legitimate way (depending on the rules of the fight)
[*]You cannot say an attack of yours hit your opponent. That constitutes as an autohit
[/list]

It is hard for the final one to be determine between both players when throwing attacks at each other. The only method is usually crafting attacks to maximize the chance of such a hit to occur but even then your opponent can block/dodge still (even if it's incredibly unlikely at times). Essentially you want to optimize the ease of use between actions you make versus what is required of your opponent and hopefully get them "out of position" where they have an extreme chance of taking a hit or no other choice but to take one.

To get into any arena combat you must know the rules of the game. Each arena has their own share of rules and playstyle so usually lurking about a bit and seeing how things work usually helps. Most players suggest getting into combat at lower power levels aka Low Tier or Realistic Melee. Honestly if you don't have any actual real life experience in fighting you should avoid RM or Realistic Melee and go straight for low tier or even Middle. There's no established upper limit for each power level so don't expect any established metric that makes any sense. You're better off thinking like this

"can my attack blow up the planet?" 
No
"blow up a continent?"
No
"A country?"
No
"A city?"
No
"A neighborhood?"
Maybe with enough collateral 
"A building?"
How tall?

This is usually a good way of rounding out exactly how strong your attack output is for a single attack (let alone ability). There's a slew of ways to bypass this with simple powers (Phasing like turning to mist or teleporting). You can imagine and even couple together your own way of estimating where your character fits but really just have fun with it. The rest is acknowledging the don'ts.

Autohitting -
You can't say your attack hits. I fire a ball of energy at you. I cannot say it hits in no turn during or after. The best one can do is try by attacking while your back is turned, laying down, distracted, off guard, you name it. 

Godmodding -
I cannot change the properties of my opponent's actions especially without just cause. You throw a fireball at me I cannot instantly go "it's fire became cure magic because I say so" or control it "I took your fireball and threw it to the floor. No sir". You cannot assume or command your opponent's character either. That's usually a [b]NO-NO[/b] in a lot of places.

Once you have these two in your mind the rest is easy. [b]Usually[/b] the ruleset in each setting is interchangeable but fall back on those two above either in name or function. They are the CORE rules of the game so players don't just disrespect each other. The third rule is 

Don't Metagame -
Your character shouldn't know about my character and what they do without just cause. Meaning if our characters are meeting the first time then it's literally a mystery to both characters who and what they are. It's easy to metagame because you are reading everything as the player so roleplaying, in essence, is PRETENDING to be the character instead. Your character's senses is important for extrapolating information your character wouldn't otherwise be aware of if the right conditions are matched.

So now you know this. Here's some general tips that may or may not help you

- Hearing, Seeing, Smelling, Tasting, and Feeling isn't just essential in real life. These are the absolute most important abilities a character can have. If they can perceive it then they can do something about it. Real simple: Slow attacks can be read. A slow build up to an attack can be read (someone lifting up a shovel to hit you with it or raising their arm to shoot you). These are we what call "telegraphs" as many put it. Your character can see things and if they are fast enough and you are prepared enough you may be able to avoid one more turn from getting hit yourself. Use this to your advantage

- Telegraphs are IMPORTANT to read. Many actions in roleplay has some physical tell (unless they don't). You want justification to evade that sword or dodge that water balloon. Always have your character observe and never turn your back on your opponent. You will also come across actions that have little telegraphs or none. You can't avoid everything.

- Positioning is important. Positioning is determined by leverage you have in a situation vs your opponent. So if you have some barrier up that blocks things and your wand has energy to attack but your opponent ran at you with a dagger and NOTHING ELSE that can be seen by your character then you are in an advantageous position. You are in a position to attack with little foreseeable penalty vs your opponent who is "OUT OF POSITION" basically not where they ought to be. Do not be fooled by this lingo; however, you can be out of position at any time even standing still or moving. Play safe.

- Detail is important. [@Doc Doctor] put it best that you don't need to BLOAT actions with unnecessary detail, but detail becomes necessary when it can determine the outcome of a fight or at least change its flow. Too much detail can bog you down though and turn this game into a chore so.. do this at your discretion, but you'll appreciate a little thoroughness from your and your opponent. There's nothing wrong with being fastidious but keep in mind it may not be wholly necessary and fun.

- Don't be afraid to taking a hit either. Sometimes you are in a situation that's just.. not nice. You don't have to take a hit if you don't legitimately have to, but in a situation where your character is literally trapped then sell it. Sell the move. A punch to the chin may not KO you, but it'd rattle you (especially to the tip that shit doesn't feel good). Do not shrug off attacks or in the term of wrestling "no sell" because NO one likes that. An attack hurts will hurt. If it's justified you got hit then take it and sell it. Reward your opponent for getting passed your tricks and ideas for defense. Just keep in mind you don't have to take a hit for the sake of fairness "I took a hit so you take a hit" because that's not how fighting works. A hit only happens because you cannot reasonably mitigate it. This is the hard part really so.. you may have to communicate with your opponent often which segways into my final point

- READ. THEIR. POSTS. Do NOT deviate from whatever detail you read in your fight. Only stick to what's written in your match. Do not make up any detail that isn't there. Do not fall for any detail that isn't there. This goes for profile sheets, fights, and actions. Read the post in the thread. That's what's happening. Think of it like a star map.

From here on just have fun. Make a main character or several of them, but love your character(s). Always use every opportunity to improve yourself and your game even if it's just your writing. Always find a creative way to do something. Just chill and play and love what you do. Eventually it'd come to you as you play so don't rush in trying to soak all of our knowledge in one sitting to try and win a single match. Just throw yourself in with someone and have a nice clean match. Get to understand the game a bit. Don't even think about being competitive until you at least have a basic understanding of it first. You roleplayed outside of the arena so it's the same engine just.. against someone else.


Edit: Also [B]TIME[/B] is the absolute factor of anything in roleplaying. It's the key component of every part of pvp so keep in mind time doesn't freeze for you or anyone else's character in game. So be very careful about Commitment. [@Doc Doctor] touched on this in a previous post so you'd be wise to read it.