I want to put a phone in my bedroom, but the phone jack just doesn't work! Is there anything I can do to like, "activate" it or something?

Trace the wiring, make sure its connected.

Pull the cover off and check to see if there are wires behind and connected. I have found sometimes that the wallplate is there but there are no wires. Next, if there are wires look for the colors. Could be Red-Green or White-Blue which are generally the first pair for inside house wiring. Follow those wires if you can. If not then you may have to open the network interface box for your house. Do you know where that is? Could be on the side of the house next to your electric meter or maybe in the basement of your home. Open it up and look for any wires not connected that are the same color. (if you're lucky they didn't get spliced somewhere else.) Just follow the same colors that are connected and connect these wires. Might not be a bad idea to mark these wires in case you need to remove them again. Now before you check for dial tone make sure the existing phones are still working. If so plug then plug a phone in the jack you are trying to get working. Hopefully you now have dial tone at your jack. One thing, if you connected the wires and now your existing phones don't work then you might have trouble on the wires you just connected. Simply remove the wires that you hopefully marked, wait for a sec. and recheck your existing phones. Then, unless you have decent troubleshooting skills it might be best to call your phone company for help. Good luck.

In addition to above, check other phone jacks, and verify all connected.
If category type, white/blue to green on jack, and blue/white to red.

By a cordless:) (Have to have some fun)!

Are you planing to have a separate line?

Actually if you do have cordless phone, go to jack in bedroom. Get dial tone on cordless,
Touch wires together, if you hear connection, you know you have the pair.