Hi Sascha, welcome to steemit.

I'll try to answer your questions, however, I will answer out of order
3.) Steem Power represents how much you have (in)vested in steem(it), in other words, it represent how much faith you have in the future of the platform. The more Steem Power you have, the higher the rewards will become. Steem power is not immediately liquid, if you want to cashout you are subjected to a waiting period and periodic payouts (see the faq in the usersmenu for a deeper explanation).
4.) Posting is mostly about quality not quantity, don't overdo it, if you post more than four times a day you should definitely keep the quality of your posts in check
2.) Steem is the token that 'represents the value of the system as a whole' where Steem Backed Dollars tries to be a peg to the USD (ie: it is supposed to be worth approx one dollar per SBD)
1.) If you are just getting started and have not invested any money out of pocket into the system I believe it is best to start out with posts that power up your steem power by 100%. This way you gain the most steem power the quickest. Once you have a decent amount of steem power it is probably wise to keep a part of your rewards more liquid. Of course this is only an opinion, mileage may vary and all.

Hope this helps,

-k