Let’s break your question down into parts to more easily answer each bit!:
How do you talk to other people?: How do you talk to other people normally? People on Tumblr are still people, of course! Just talk to them the way you would talk to somebody you were approaching in chemistry class to ask if they would like to be your lab partner. It’s almost the same thing – you’re two people working together to create something that neither of you could have created alone, and that will benefit both of you (and if you get it right, you produce great chemistry!). ;)
How do you write a para?: Do you like to write fanfiction? Do you like to write original works? A lot of roleplayers I know enjoy doing writing on their own time as well as the kind of writing they do in roleplay. Just take what you know from that, and apply it to your roleplay, keeping in mind that you only control one of the characters, not all of them, so you can only write out the actions of your character. However, this does not mean that you should only react to what your partner has said – it’s as much your responsibility as it is theirs to move the plot along!
If they give you a little, give them a little more. It’s up to the two of you to get Dorothy and the Tin Man to the Emerald City, and you can’t do that if you’re both still stuck describing a picnic that’s been going on for the last three days! One of you has to find the ants on their sandwich and decide it’s time to pack up and go, and if it seems like your partner isn’t going to be doing it anytime soon, then it’s up to you to bring the thunder, or the ants, or whatever it is that finally gets Dorothy and her little dog, too, up and going again.
Sometimes you can do this with one paragraph, sometimes it takes several. The more you write, the more you’ll be able to decide whether a response needs to be longer or shorter. It takes practice to be able to do both, though, so don’t be discouraged if for a long while you can only do short para responses, or only long ones! Lord knows I’ve been struggling to cut down on the length of my responses – for a long time my poor Tumblr partners had to snooze through novellas to get to the point of my posts! Practice makes… well, not perfect, admittedly, but better! ;)
How do you do everything?: Well, first you need three large eggs, a cup of water, and a cup of cooking oil… or did you want to learn about Olympic ice skating first? I’m just kidding, but like any skill you pick up in life, roleplay takes a little bit of courage and a lot of practice, but eventually it will feel completely natural! Just go for it! Your first couple of roleplays will probably be shaky as all getout, but isn’t that how everything is? My first violin lesson sounded like dying seagulls, but I kept playing anyway, and now I teach! Just GO FOR IT – what’s the worst that could happen? You learn from your mistakes, and then you come back and do it again, better than ever!
And who knows, you might even make some great roleplaying friends along the way. :)