Sounds like only one cylinder is firing. After it has been running a minute or two, spray a little mist of water on each exhaust pipe near the engine. If the water immediately sizzles and evaporates, that cylinder is firing. If the water is much slower to sizzle and evaporate on one pipe, you will have identified the problem child.
Pull the spark plug from the non-firing cylinder and reinsert it into the spark plug cap in a way that allows the spark plug threads to have good contact with the engine, preferably away from the spark plug hole. Try to start the engine and look for a spark. It should be blue or bluish white. If there is no spark, or it is yellow and weak, the problem will likely be in the secondary side of the ignition. If there is healthy spark, the problem is elsewhere.
If the bike has been sitting unridden for a while, the pilot circuit in each carburetor is likely clogged with dried fuel.
If the carb holders are old, hard and cracked, they may be leaking enough to cause a misfire.
If the carbs have recently been removed and reinstalled, they may not be fully seated in the holders.
Rick’s advice about the routine maintenance is good, especially on an old machine that is new to you. Valve adjustment is often neglected and can cause a variety of problems if ignored too long. Performing all the routine maintenance will eliminate potential problems, help familiarize you with the machine, and will help the bike run its best.