Various ways, mostly you'll be getting the ones for your total like and post counts, and one every year (for 2 or 3, I think?). Others, you have to either been apart of an officially run contest or other random misc. stuff that you've more than likely missed out on the potential of doing.
Terraria Community Forums
Techhunter_Talon
Gearzoid II
Trophy names?
Kazzymodus
Here's a link of a list of all the available (and unavailable) trophies.
Xag
@Gearzoid II yeah,@Kazzymodus where ? lol