Istalling new tach

i want to install a tach on my 289 automatic. how would i go bout doing that.

Do you mean factory or aftermarket?...Can't help you with factory but aftermarket is very simple...Normally you have a red wire that needs to go to a switched 12 volt source..One black wire that goes to a ground..One white wire that goes to your interior lights or can be hooked in with the red wire if you want the light on any time the key is turned on...And one green wire that you hook to the negative side of your coil....The tach can easily be mounted to your steering column with a big hose clamp....Most tachs have an internal switch that needs to be set for your engine size (4-6-8 cylinder)..

If you have a 67 68 Mustang then you could get tach from these guys that goes in the clock space.

Mustang Mini-tach

i should have been more specific. Its a 68, and did not originally come with any wiring for a tach. Will i need some sort of sender unit, and where will i need to install it. i was planning on puttng aftermarket gauges on it.

No sender, just like Frdnut said to the negative side of the coil.