RTK GPS in the use of APM

can anybody tell me what kind of RTK
can be used in the APM? can anybody tell me
a website of the RTK?
thank you!

When you say APM, do you mean ardupilot in general, or the APM 2.8 autopilot board specifically?

APM 2.8 does not support RTK, but any board from pixhawk does. The two options are the emlid reach RTK system, or the hex here+ RTK kit