Usually you have a Source Book, either the DM will tell you which one he is using or explain stuff.  Most of this stuff can be found free online.

A lot of the +1 or +3 or whatever comes from picking skills for your character or finding gear.  You might have an Exquisite Rapier that adds +1 to thrusting attacks and have the skills Fencer which also adds +1 to thrusting attacks.

So youd take the damage your weapon does (as found either in the Source Book the GM says you're using or by his explanation of the weapon) and add the +1 from it being exquisite and the +1 from your skill.

Usually you would track these things on a character sheet.

Ex of weapons
https://roll20.net/compendium/dnd5e/Weapons#content

Ex of character sheet
[img]https://www.dmsguild.com/images/8957/237888.jpg[/img]

Part of the GMs job is to give you the information you need to come up with a character.  Tell you which Source Books you can pick from and they should be providing you with links where you can access enough of that stuff to faff yourself up a Character Sheet.