Hey there! Welcome to Guild.

From what it sounds like, you're looking for what we call a 'Jump-In' RP here. You can find those by looking at an RP's 'Join Status' which is located at the top left-hand side of the page under the RP's Name and GM. If the 'Join Status' is 'Apply', those RPs are looking for you to submit a character sheet, but if the 'Join Status' is 'Jump-In' you should be free to just dive on in.

I've never done a 'Jump-In' RP myself, I'm a bit of a schemer so I like my character sheets, so I might not have the ins and out exactly right but I'm sure the GM of any 'Jump-In' RP will be more than happy to clarify any questions you might have!

Best of luck finding what you're looking for and once again, welcome to Guild!