Okay, this makes it slightly harder then if you were just a player, but that's totally okay. A lot of advice can vary wildly depending on what the type of rp is (i.e., is it a linearly told story rp, or a sandbox rp, etcetc) but a few general hints and tips can help. 

If it's a story focused rp, having some ideas in knowing what you want to occur in the future can help pay off in dividends. Having a goal or general idea of where you want to take and lead the rp is important, as it'll allow you to write content foreshadowing these things and help give the content of the IC and your GMing some direction. With my own rp I often consider what I am leading my players to and where the story will be heading, thus I can create events and introduce new NPCs that'll help propel things forward and give my players things to interact with and react to. 

And that's the most important thing: Interaction (and reaction). Think of what types of things can benefit your players' characters from an interaction viewpoint, and ideas can help stem from there. For a very obvious example, one of your players has a hotheaded character prone to explosive anger. Set up a situation that could capitalise on such a thing and result in them becoming angry. As the rp goes on and you get more confident, you can subvert these types of things too by offering situations that are slightly more unusual for those characters and out of the box, that goes out of the norm of expected reactions, which can help build their character's development in surprising new ways. Not only that, but knowing what the particular tastes and interests of your players are can help a lot as well, from the major to the minor. I had a player who was interested in music and fashion, and I incorporated that into the rp by offering short scenes of clothes/outfit buying and song performances etc. Another player was interested in wrestling, which grew into its own storyarc with the characters becoming embroiled in a wrestling match. 

Finally, communication. Never be afraid of communicating with your players. The most amazing part of rping for me is that I get my inspiration for writing, and what happens in my writing from my players. Talk to them and ask them what they'd like to see or do. If you get stuck and need some help or suggestions, you can always ask them. It helps get them more involved and more invested in the rp. RP's are a group effort, and that includes every player making an effort to push things on and not just the GM. If you need any further help with main story/plot/rp progression, you can always consider having a co-gm who can help you make important decisions. 

Hopefully this helps in some small way. I'm sure others here could also contribute some suggestions too!