If your bike has been neglected for a while, it could probably use some TLC! If you take it to your local bike shop, the shop mechanic will check your brakes, make sure your wheels are true, the tires are in good shape, and that your chain is lubed and gear shifters are adjusted. They should also help you make sure your bike seat is adjusted properly.
Then if you don’t already have them, there are three essential additional items to get. The first is a helmet! Helmets save lives, don’t ride without one! The second item is a pair of bike lights, front and back. Even if you don’t intend to be out when it gets dark, something might affect your plans and you’ll be glad to have them. The USB rechargeable lights are great. The third thing to get is a bike lock. No lock is fool-proof, but any lock will help prevent someone from just walking away with your bike.