Welcome :)

1. Entirely depends on the roleplay that you join. See the thread's opening post to get the gist.
2
a - Above poster says the same as I would.
b - Express interest, ask questions, and ensure the roleplay is something you want to invest in before putting the effort into making a character for it. It doesn't reflect on you too well if you find out later, after joining it, that the roleplay just didn't fit. There is no shame in noting that you're interested but intend to wait and see what else occurs.
3. Casual forum ones tend to operate between once a day/once a week if they're healthy. Free forum, multiple times a day or once a day consistently. Advanced forums, closer to a week or more. That said, there's a big mix that depends on the GM. Past the week mark and the health of the roleplay is questionable if it isn't in the advanced forum.
4. Most threads have an "apply" tag in green, in which case they mean you should post a character sheet first. Sometimes they're lazy [s]like me[/s] and may end up forgetting, which makes it a safe bet to post openly and ask if there's spots open first.

The exact policies of each roleplay are, as noted above, dependent on the GM. Feel free to shoot me a PM if you have more Q's.