Sure, I can try to help. The roleplay system can indeed be tricky to navigate, so I'll explain things as best I can to help you out.
There are a couple places to go looking for roleplays to join depending on what you're after, most of them listed under the Roleplay Search header on the front page of the site. The
Interest Checking & Creation area is where people looking to make group roleplays (groups being anything with 3 or more people involved) go post ideas to see if people are interested or to collaboratively create a roleplay with others; you can look through existing threads here to find a group roleplay to join in its early stages, or you can post your own interest checking thread for your own roleplay if you so desire.
Group Roleplay Advertising is similar, but it's meant for roleplays that are already running and looking for more members, so you can find older roleplays here to join if you don't mind hopping in after potentially lots of things have happened in the roleplay already.
Seeking Group Invites is a place to post threads to more passively look for group roleplays to join, where you say what sort of roleplay you're looking for and hope someone comes along to tell you that they're making one or already have one that fits what you want.
Seeking One On One Partners is where all the interest checking and advertising for one on one roleplays happens. You can cruise the existing threads to find something interesting, or you can make you own thread with lists of plots or pairings or whatever you're looking for. Note that if you're looking to join or make a roleplay with adult content you should be using the Libertine subforum of the above listed sections, because looking for that sort of thing elsewhere can get you in trouble (which you hopefully already knew after reading the site rules, but reminders never hurt).
Alternatively, you can go look through all the various roleplay genre sections to find existing group roleplays to join (this method is silly for one on one things because all you'll see is roleplays where there are already two players, so sticking to the seeking area for those is your best bet). The way roleplays are laid out can be confusing, so I'll give you a brief overview of them. When you enter one of the genre sections of the group or one on one roleplaying areas you'll see a bunch of roleplays listed with some basic information, and off to the left of that list (or below if it you're using a device with a small screen) there is a box that lists the different categories and makes for quicker navigation through the genres. When you click on a roleplay to check out you should be taken to its Overview page, which should ideally give you an overview of the roleplay as the name suggests. Under the roleplay's title and icon (the default is a grey and white image of gears, so if you see that it means the creator did not add an icon of their own choosing) you should see some tabs. There might be tabs named Rules and Cast List, but there might not be; these are optional tabs that the roleplay creator has to put information in before they will be visible, and they ought to contain just what it sounds like. There will always be a tab named Thread, and that's where things get a little tricky because Group and One on One roleplays are laid out differently from one another. When a One on One roleplay Overview is created there is also an in character thread created that is attached to it; the Thread tab for 1x1 roleplays will always be meant for doing the actual in character roleplay stuff, and if a thread for out of character talk is wanted that has to be created separately and can be linked to the Overview but will not create a new tab (though hopefully some day we'll find a way to make this possible). When a Group roleplay Overview is created the Thread tab will be an out of character thread, so the in character thread has to be made separately and linked to the Overview. Extra threads linked to Overviews should show up as buttons to the right of the screen if you're using a normal computer, or I believe they're right at the top of the page if you're on a phone or something.
Holy crap that was a lot of stuff. It's a sort of complicated system, but after you fumble your way through things a couple times it starts to make sense. Let me know if that didn't cover what you were confused about and I'll try to cover that too, probably also in excruciating detail because that's kind of my style.