How often should I have maintenance for my bike?
Have it looked over once decade, whether it needs it or not.

Otherwise, if nothing's giving you trouble, all you need to do is to air up the tires and put some chain oil (buy "dry" chain oil from a bike store) on your chain once or twice a season.

Assuming this is a multi-speed bike, to oil, first hold a rag on the chain and crank backwards to wipe off as much surface grime as possible.  Also wipe off the derailers.  Then slowly crank backwards while dribbling oil onto the chain.  Go over it 2-3 times.  Then wipe the chain again -- you only need oil on the INSIDE of the chain.  Finally, put a few drops of oil on the derailer idler pulley bearings and on the derailer pivots, and, again, wipe off any excess.

An occasional squirt of spray lube (eg, TriFlow) into the shifter/brake cable housings is also a good idea.

Be sure to check your air pressure at least every 3 days, or before each ride if longer.  It's a good idea to get a "floor pump" with a built-in pressure gauge -- much easier to use than compact pumps, and you'll not be so inclined to put it off as you would if you just use the gas station pump.