Hey [@Muscleshroom]! It's great to see you willing to get to know the place. To be honest I'm terrible at finding my way around websites but once you know what you're doing navigating around here is a breeze

When applying to a roleplay, do you just PM the creator? 
Generally No. If you're interested in the interest check section add a comment. If it's in the actual RP section above all the comments is 3 tabs. IC, OOC and Character. Best to read through the first post in the OOC to check the GM's wanted method to apply. If nothing is said just drop a comment into the OOC claiming interest. The only exception is if the GM tells you another way to do so. I have seen one or 2 cases where the GM asks you to PM them but this is rare unless you're in the 1x1 section

Is there any specific format to PM them in or do you just ask? 
As for format, usually not, they may specify so in their first post

I noticed the character section, should I have a thread there to post my characters, or is that optional? 
This is totally optional

How important is it to be active on the more casual sides of roleplays here? 
This depends on the GM. Some state you must post so much a week while some are more of a 'whenever you can'. Talk to the GM about it. We understand life happens so if you're in a RP with posting standards and you have to disappear just shout the GM a message in the OOC or PM them to let them know

Hope this helped :)