Carp are kind of wimpy and unaggressive in the current version. But if you really want to trap them, here's what you do.
First, you need to be on a map that has a river with Carp in it. You can't get them from traders, you need to catch them from the wild.
Dig a canal next to but not connected to the river. Line the bottom of the canal with cage traps. Dig the canal one tile further and place floodgates there. Build a lever somewhere else and link it to the floodgate. Now channel out the final connection between the other side of the floodgate and the river. Water should not fill your canal until you pull the lever to let water into the canal.
Now you wait. Eventually, assuming your embark site has carp in the first place, carp may wander into the canal and get caught.
Pull the lever to close the floodgates. Now build a pump and pump the water from the canal back into the river. You can now retrieve your caged carp.
You won't be able to tame them without modding, since carp don't have the PET tag. And even if you do tame them, aquatic creatures aren't much use for attacking anything, and carp aren't all that dangerous anymore.
You may also look at this thread for some ideas and inspiration:
http://www.bay12forums.com/smf/index.php?topic=75780.0