Several ways.
First, determine if there really is a weight and balance discrepancy with a proper weight and balance calculation. If the aircraft is overweight and out of CG, can you simply remove some baggage or other non essential cargo in order to return to the weight and balance envelope? If so, then do this, perform another weight and balance calculation and if it is within the envelope for safe flight, problem solved.
If the airplane is within gross weight but outside of CG, can you move some payload around to another fuselage station e.g. A fwd baggage compartment, etc so the CG can be moved within limits? If so, do that, then do another weight and balance calculation. If you're within the envelope, you're good to go.
If you cannot lose or rearrange any other non essential baggage in the cabin and you're still overweight, first verify that removing said amount of fuel will 1) keep the aircraft within weight and balance limits and 2) will be enough to safely complete the flight with the required onboard reserve, you have two basic option, drain it out, siphon it out or burn it off.
Before beginning this exercise, make sure you have an accurate means to gauge just how much fuel is in the tanks currently. The tanks are generally placarded with their useable capacities or this will be listed in the Pilot's Operating Handbook for the aircraft in question. Sometimes takes also have pre-marked indicators inside the tanks where a known quantity of fuel will level with these markings or metal tabs. Some firms also sell gauges for determining fuel quantity in the tanks; just make sure you have one that's approved by the manufacturer for your make and model airplane. DO NOT use the cockpit fuel gauges in a light GA aircraft to determine onboard fuel quantity; by regulations these are only required to be accurate when they read as empty (how convienent).
You could drain fuel from the tanks through the sumps, generally located under the wings. A tool with a rod would be required to open the valve on the sumps and you could use a tube with a funnel on it to collect the fuel into a container.
Siphoning the gas may be easier using a tube through the gravity filler ports on the top of the wings. One downside is that these filler ports are often on the highest part of the tanks and may require a long hose to siphon the gas.
The third option is simply to burn off the gas by making a flight prior to the arrival of the passengers for the planned trip. Using the takeoff, climb, cruise and power settings charts in the Pilot's Operating Handbook, you can make an accurate estimation as to how long it will take and what power settings to use to consume the excess fuel prior to loading for the trip.
