I am a new guy who needs to know how to play

I'll answer you as the RP Executive, good qusstions.
1. Depends on the RP of course can take a few hours or a few months.
2. Yes they can, but I suggested a rule while ago that has been now largely accepted and adopted which is that you can't attack a person multiple times until the defender responds or 24 hours pass of the previous attack. This doesn't apply if you attack with a different force though. Also, you may always request a ceasefire.
3. Only for spoilers are to be seen only by the one(s) the poster says. Other players can't open them in any scenario unless authorized. There are no spies.
4. Depends on the RP. You generally take casualties in all RPs. There isn't a system for calculating them nowadays, they're upto your and other players' consideration.
5. You can build fortresses according to the RPs rules.
6. Depends on the RP.

There are many types of RP that have been played or suggested before, but to mention the ones that have been played recently, these could be Diplomatic War, Army Group and TW (Theoretical War). But there are RPs that do not belong to a certain type, which is more common. In general, every RP usually has certain rules (to keep the game fair) and specific rules for that RP (like if betrayal is possible).

To mention these types simply:
Army Group: The most important difference of Army Group from other RPs is that the players manage the armies of the same countries as army groups, like a team game, other than that the rules may change in each RP. An example for this RP type, which is complete: european-war-4.boards.net/thread/16205/ww2-1942-army-group-rp

TW: The rules of Theoretical Wars also depend on the game, there is no fixed set of rules.

Diplomatic War: Diplomacy is at the forefront in the Diplomatic War RPs, as the name suggests. There are two main alliances, they start the game on a fixed point. In the game, the creator regularly creates events, players react to these events and get points based on their reaction. Wars only exist if the event allows. The alliance with the most points wins the game. If you want to look at this type in detail, here is an example, which has been played before: european-war-4.boards.net/thread/15202/diplomatic-wars-1

Note: By the way, if you would like to take a look and join the ongoing RP, here is the link of the thread: european-war-4.boards.net/thread/16769/spanish-civil-war-rp. New players are always welcome!