How hard is it to install subwoofers in a car?

I already have a nice pioneer cd player and nice pioneer speakers, but I want subs... is it hard to install? I need an amp with sub woofers right? i just have a little honda civic so I don't want anything too strong, but just enough to make my rap and rock music b sound good, right now it just sounds weak and wimpy. Any help is welcome...any good websites, too.