RP is best learned through practice. Reading good books (Have you tried His Dark Materials?) will also help you with developing characters and learning how to express them. And of course, a wide and colourful vocabulary can be quite useful.
I can give you a few tips though, that I have picked up:
All good roleplays start with an interesting character. It's near damn impossible to enjoy an RP fully if you play someone that won't help fill the storyline.
In my opinion, a character should have at least one feature (a talent, like being direct and outgoing), one flaw (that handicaps them, like being socially underdeveloped and challenged) and one bit to their past that will affect them to this day (like living alone in a small abandoned space station, which leads to social anxiety, fear of large crowds and thundering sounds that may sound like aliens attacking).
In addition, you need to think about their body language and manner of speaking. Don't be afraid of having notes around to help you remember, I have a notepad file for every roleplay I've done online. And if you want to develop as a roleplayer, then give yourself a challenge. Have them speak and act in a way that is not natural to you. If you are an outgoing person, play someone that is quiet. If you don't like to be touched, play someone that always puts a hand on someone's shoulder or rubs their head.
And let your character develop. Let their experiences change them. The examples of characteristics above, are from one of my characters and she lived alone her entire life. As a result, she could barely speak. Not because she was dumb, but because everyone else died when she was three. This let me develop her language skills as we went along and I LOVED doing it. At the beginning, she'd associate anything that hurt, was scary or evil, as "bad". Like that hallway was bad, that man was bad that warm coffee was bad. Simply because she had no other word for it. As the story went on, her vocabulary developed and I began using more words as she heard them. I didn't keep a record of this, I just used my memory. But it did make her entertaining for me and others.
Secondly, you need to know how to express yourself to your fellow roleplayers. I like to imagine every scene as puppets, and you are in charge of your own puppet. Your character won't do anything unless you say so.
Imagine two people; a man and a woman. The roleplayer wants his character to hug the other in an embrace:
*embraces her lovingly*
Well yes, the puppet has now opened his arms, bended forward awkwardly and wrapped them around her. But it does feel a little naked and empty.
*steps forward and takes her in my arms, embracing her lovingly*
Now the entire body of the puppet is in on the action, which makes it come more alive. However, it doesn't say anything about the puppet's mood or what he's thinking.
*flashing a bright smile, I step forward to her and cradle her into my arms in a warm embrace, my own eyes welling up in tears while gazing at her perfect face and I whisper softly in disbelief*
"I cannae believe you made it, lass..."
In other words; body, voice and mood. If you character doesn't say anything, then the other two are even more important. I find this a great and easy way to make the roleplay a bit more colourful. You could also describe how the wind tosses her hair around and how you use your free, rough hand to pull it away, touching her porcelain skin, only to feel her own tears running as well. The options are endless.
I'm not saying I'm a great roleplayer, but I hope this helps to some extent.
