• A quality U-lock and maybe a cable lock to lock your wheels as well.
• Front and rear lights.
• A water bottle cage if you don't have one.

Those are the only things you will need. But there are a bunch of other goodies you could buy for your bike. Do you use this bike to commute? Or is it for fun/fitness?

• Racks and bags to carry stuff.
• A helmet.
• Mirrors.
• Bell/airzound
• Reflective tape or vest
• Bicycle computer

Etc.