My 2001 tahoe cranks over but doesnt stay running, i put a new starter in and new ignition..Ive tried bypassing the security system,,PLEASE comeone that knows this particular vehicle help me get it started.

Why? If it cranks.
Exactly what did you do?

Let'* get on the same page with terminology:

Cranks: The starter is turning the engine
Starts: The engine starts to run without assistance from the starter

Does the engine start? . . . or does it only crank?

I agree with carfixer007:

If it cranks, then likely you don't need to replace the starter.

What was done to bypass the security system? Are we talking about the GM built-in security system or some kind of aftermarket security system?