There doesn't seem to be one in my truck.

What kind do I need?

 

Personally I had a spare one from a superduty for my 88 F150. It works fine I also have another one from a ranger in the 90 F150 that worked fine today.
Trav

 

It is under the hood on the right behind the battery I believe. If it isn't there just grab a basic bottle jack or scissor jack and toss it under the seat. I don't know how big you truck is but base the size of the jack on the size of your truck. Screw type jacks are fine for emergencies, but a pain to use regularly, so if you intend to use it as your primary use jack for the truck and other vehicles I highly recommend going with a hydraulic jack.

 

As mentioned above, depending on the year, the jack and tire iron are under the hood on either side of the engine. On my '89 the tire iron is on the right (looking at engine) and the jack on the left. Yours is probably the same. This jack is fine for flats, but as suggested above, if you are going to be using it for repair work on a more regular basis, you will probably want to pick up a more sturdy 2 or 3 ton pump jack depending on the size of your truck.