You definitely want to have all your cards in the system before you start trading with people. This should include descriptions of card quality if you have any cards that aren't mint/near-mint.
A lot of people prefer you message them first before you initiate a trade. Personally, I don't mind if people send trade offers without talking to me first and I will often accept them if they are fair. One thing that is really important to remember is that once you send someone a trade offer, that is a binding agreement and if the other person accepts, you must follow through with the trade as you offered it.
As far as shipping goes, you will almost always have to send your part of the trade first since you are new. Because of this, you want to be careful that you are only trading with reputable people that are established traders with many references. After they receiver your part of the trade, they then send out their part of the trade, provided that they are satisfied with the condition your cards are in.