Okay, we need to start somewhere, so let's review the basics. Internal combustion engines require three things to run:
# Air[br]
The air part usually isn't the problem unless your air filter as been filled with dirt by an industrious squirrel. However, with the advent of mass airflow sensors and throttle bodies, significant air leaks between the filter and the manifold can cause problems.
# Fuel[br]
On fuel injected vehicles, the individual injectors need power to turn on, and there needs to be pressure in the fuel line, so both of those things have to be checked.
# Spark[br]
The general method here is to pull a spark plug, reattach the wire and ground the tip to the engine block. Crank over the motor and visually check to verify that a spark is jumping between the electrode and the tip of the plug.
Of course, with the addition of computers in a vehicle, that system can itself keep any one of those three things listed above from working. However, it also comes with diagnostics in the form of the OBD-II port. If you have or can borrow a scanner, reading those codes can give you valuable information in diagnosing why your Tahoe won't start.