There are some basic things to know about them before you buy one; there are two types - those that oscillate their bristles (Oral B), and those that vibrate (Phillips Sonicare).
The type that appeals to you is completely a personal choice. Both are well reviewed and known to work well for cleaning, although the best toothbrush is the one that you use best.
If you go the oral B route like I have, the biggest difference among handles will be how fast they oscillate. A higher number here is better. Everything else is useless marketing (lights, colors, screens, etc.). You don't need any of that because it's redundant to the timers handles have built into them, where they momentarily pause every 30 seconds to indicate that you should brush the next quadrant of your mouth.
Note that these brushes tend to be 'buy it for life' products. I've seen people use the same handle for 15+ years, and even the cheapest one oral b sells is made well.
I'd recommend buying a lighter color handle though, because there's a well-documented problem with these handles developing mold over time. Buying a black handle would leave you oblivious to it if you have the issue, and you'd never know to clean it. A good way to avoid mold is to keep the head separate from the handle when you're not using it. I 3d printed a stand to hold the handle securely, although it's able to stand up on its own just fine.
Finally, I'd warn you against third party brush heads. I tried buying a pack from a seller named 'Maycreate' on amazon, and they were significantly cheaper. I use the word 'cheaper' in both its senses; the brushheads wore down extremely quick, and developed a groove along their path of rotation such that they no longer stayed straight while rotating. I think Oral B's official brush heads are way better.