Go to a local bike shop and you'll end up with a quality, comfortable ride that will last for years as long as it's properly maintained.
Do not buy anything from big box stores because those bikes have junk components and will not last very long.
I'd suggest a hybrid, like others did above, which will probably cost around $400-500. Employees can help you with sizing - and yes - they do make adult bikes for short people!
I have thought about taking my little guy with on rides on my road bike but don't think it's very safe since I average 20mph and mostly ride on rural highways.
I'm a triathlete so I'm very passionate about bikes
