The best bike computers on the market that I recommend are the Garmin computers. They’re fantastic. The Garmin 500 or 510 are a really good entry level computer. You want to buy them with a speed and cadence sensor option. They have heart rate, GPS, hook up to Strava, and you get a good indication of where you’re going with your training.
If you don’t have a lot of money, then you can just get a little bike computer that’s got a cadence meter on it. It’s hooked up to a magnet.
Get your bike shop to sort it out for you. If you buy it from a bike shop you can get them to fit it to your bike and that’ll give you cadence (RPM).