bro usually all mobo have small marking on every plug/slot on its board.. look carefully an identify whether it is for 8pin cpu, 24pin mobo, sata, cpu fan, etc..

read the manual given and it should be easy.. regarding with the thermal paste, u can clean it up with any alcohol base liquid.. just go to any hardware store or pharmacy and ask for alcohol wipes/liquid.. if you lazy to find, just wipe it with anything and make sure no any residue left on it.. then put a pea size of new thermal paste on the cpu and mount the heatsink..