Stereo & Electronics - crossover question

so i'm a stereo nOOb and know nothing about electronics. that being said, i currently have rockford components in my front doors (98 TA) but no crossover so the tweeters are just hanging out. what do i need to look for in a crossover, found a couple on ebay:

http://img.auctiva.com/imgdata/4/6/5/4/0/3/webimg/54947009_tp.jpg

OR

http://electronicgiant.com/catalog/images/ROCKFORD_CROSSOVER.jpg