The first thing I was taught when I got into RP was this: Confidence, Vision and Writing.
Confidence - You have to believe. Believe in your ability to create and most importantly, you have to be able to believe in yourself. Even if you don't have a great amount of self-esteem, in the RP world you need to just let it all out. Everyone has a different level of experience, a different style of RPing and a different way of expressing themselves. Nobody will be out to judge you as long as you have the confidence to just go out and do something you'd usually never do as a writer. Don't be afraid to take things to another level and go beyond your comfort zone.
Vision - Sure, anybody can say that a character is happy or sad but you need to be able to show it through your words. Showing detailed emotion or just detail to the surroundings around your character itself, you need to be able to make the world around you come alive and make it believable. Tia is an excellent example of someone who is very strong in showing emotion with her characters, she's able to show the tears running down her face, throwing herself around when she's upset, detailing her expressions through any emotional status her characters feel regardless of happy, sad or angry. Example, if your character is outside walking around and the sun is shining, just start with something small to get things rolling.. like the sun reflects brightly off of his dark blue shell as he makes his way into the forest. Improving your vision as a RPer really just takes practice by starting on the small details and observing the ability of detail that the RPers around you use. Keep working off of these and your ability to work in details into your own characters will continue to improve and deepen.
Writing - In terms of making your characters of believable, this is the most important skill to learn. It isn't just in your grammar, punctuation or choice of words but it's also connected to writing plots, stories and giving your character a background that isn't too over the top.. something that people can believe in and relate to, in other words. The fellow we had RPing Mario years back, he was a strong case of the opposite. He couldn't be touched and made Mario to look like some overbuilt superhero, it wasn't believable which rubbed off onto the rest of us and eventually grew a strong hatred for him because of his writing ability and perception. The plots and stories you make, they don't have to be anything epic or complex.. if you're not exactly the detail oriented, start it off small and make a basic plot such as "koopas building a time machine and needs a piece for it located in the Vanilla Dome." In terms of a backstory for your character, it is the same as writing plots.. don't overbuild it and make it believable. If you're not good at writing at a history/backstory, keep things simple and just practice and observe those characters around you. The more you practice, the better you are able to build your story.
We're all here to help you, never be afraid to seek advice. I was an inexperienced RPer when I first got started, I never even tried D&D. But this was the advice I got from my tremendous friend Quetzal back in Dragons of Lightning Mountain almost 30 years ago and as a friend offering advice to next generation, I have now passed this advice onto you. I know I'm wordy, but I am telling you everything I know. Now go out and have some fun!