All good answers already. I would only add that when you say “Amen” you are acknowledging that it is indeed the Body or Blood of Christ. In essence, you are saying “I believe” to Christ’s real presence.
As a somewhat recent convert (February 2010), I appreciate how important this is to you and therefore your desire to get it right. Whatever you do, don’t beat yourself up for any mistakes but focus instead on what is actually happening at Mass and the wonderful gift we receive in the Eucharist.
You might want to see if your parish offers any special “refresher” courses for returning Catholics. If they do, it may be something that you would find really helpful. If not, you may want to sit in on any RCIA classes. Speak with your director of religious education, explain that your are returning and ask for their advice.
Lastly, just in case, if you have not been to confession, then do so immediately. I strongly recommend making an appointment with your priest for your first time back (and don’t worry, it will make his day).
Welcome back!!!