You have to look into the user's manual of your AVR/Amplifier. It will say with which cartridge is compatible with (MM = Moving Magnet or MC = Moving Coil).
The cartridge (or pickup) is the part you mount on the turntable's tonearm.
The choice is up to your tastes (ask the dealer...
)
Plus: if it has a Phono IN (the AVR/Amplifier) most likely it's compatible with at least an MM cartridge (usually MM pickups have a little more output power and they don't require a turntable pre-amp if a Phono IN is present in the AVR).
For example: I had an Yamaha AVR (21 year old) that was compatible with both MM and MC pickups and had a Phono IN. My new Denon AVR-7200W has a Phono IN but it's compatible only with MM pickups. Since my Technics SL-1200 has a MC Ortofon Cadenza Bronze pickup, I had to buy a RIAA/step up transformer.
There are many different type of transformers/RIAA/pre-amps (and different prices).
I got myself a Cambridge Audio 651P which can be used with both MM and MC pickups (has a selector switch in the back) and works pretty damn good.
Just remember that the line out from the turntable pre-amp MUST NOT be connected to the Phono IN of the AVR (even if present); it must go to any other analog in (i.e. AUX, CD or whatever...)