You should, as you put it, "hook the fish." It's more properly called "setting the hook," and it's part of the process of fishing. Some fish, like bass, will help you out and set the hook themselves by hitting aggressively. Other fish, like trout, will take your lure or bait very carefully, and will spit it right back out without hooking up. In most casts, at some point, you will want to pull your rod back and make sure the hook sinks into the fish's mouth.
There is different timing to setting the hook. Sometimes you want to set the hook within a second or so -- like when a fish takes a dry fly from the surface. Sometimes you want to wait a few seconds until the lure/bait is really in the fish's mouth, like with jigs and worms.
With catfish, sometimes it seems like they sort of "play" with the bait before they take it. They suck it in or sniff it or something. You feel them at the end of the line, but if you set the hook too fast you'll pull it away from them. If a catfish is onto any sort of edible bait, they'll take it eventually, so don't be hasty -- wait till you get a positive pull (not just flittery little tugs), then cross his eyes.
