Listen to what Tod Daniels tells you, he knows his stuff.
One thing I will add is that you should inform your immigration office where you are staying within 24 hours of your arrival in thailand.
The first time I came over I did not do that, and when I went in for the extension I was questioned as to why I had not informed them. I got the extension ok but they said if I come again I must do it.
That will be the same as the TM30 that Tod mentioned I assume.