I tested one of my computers i started with minecraft and fps was very low i don't know why my setup is: fx 6300,rx560,8gb ram?

Did you check to see if your hardware isn't over heating? If it is, then it (CPU and/or graphics card) is probably slowing itself down. Have you also made sure your drivers are up-to-date? Have you lowered the graphical settings of the game(s)?

Well, rx 560 isn't a powerhouse gpu by current standards. Look up the recommended gpu for minecraft, then compare that to the one you have using the comparative gpu table at a gamers' resource such as tomshardware.com

Did you check how much ram you can actually use? And have you checked if you actually put the GPU to the rx560 and not the processor? (sorry for my bad grammar i'm not from england or from the US)

Minecraft is extremely CPU-oriented, and the AMD FX-6300 is pretty outdated and slow. Upgrade to Ryzen 3 (which means a new motherboard and RAM, also).