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I would suggest checking out some of the interest checks under "Recruit Here" and see if any interest you. Then you can message them on the post or private message them (they'll usually say what they prefer on the post.)
1x1 is what it sounds like. You roleplay one on one with another person.
Group is for more than two people.
Dice is for RPGs like D&D, Call of Cthulhu etc.
Honestly, I'm not really sure what Quest and Nation Building checks are.
I think Off-Site is for if you want to meet on Discord or something to RP instead of in RPN.
You can also create your own interest check.
Usually interest checks include:
Any rules you have
What kind of POV you like to use. 3rd person past tense is very common.
The length of posts you like to have. Anywhere from one-liners, 2-3 paragraphs, to 1500 words each post.
What genres, fandoms, tropes, etc. you are interested in.
If you have any plot ideas.
Stuff like that. If you are interested in creating more fancy posts, I would suggest checking out the BBC Code section and learning how to code a little.
Just to fill in the gaps on that excellent last answer:
Quest RPs are essentially Choose Your Own Adventure stories, where the players vote on what should happen next. Nation Building RPs usually involve entire countries, armies, peoples, etc., rather than a party of individual characters like in traditional group RPs.
Make character sheets. Get an avatar! Read some roleplaying forums until you understand roleplaying. Find a roleplay you like in interests! good luck! have fun!