What a Vague Post, You might want to start by telling us Why you need a Harddrive, New Build, or maybe your old one died, Or prehaps you just want extra storage?.
Which in this case it seems you want to replace an old one.
The next "thing" you should Do is list the specifications of your Motherboard and PC.
But For the most part it is a simple process, Use Same Cables, etc etc. You will have to install an OS and backup information that you have from the old HD.