In order to give you the exact steps you'll need to follow we would need to know your motherboard make/model. But really, if you tell us that all we would do is download your manual and read it, so you may as well do that yourself.
The standard steps are:
1. Boot up in safe mode, find the onboard graphics in device manager and disable it.
2. Reboot system into your bios
3. Find the settings in the bios to switch from onboard to PCIe "init boot device" is a common title for what you're looking for but it varies.
4. Shut down the system and install new card. Don't forget to give it a power supply connection if it is required (some do, some don't, your manual will tell you).
5. Connect monitor to new card, and boot up. It should at this point automatically detect the new graphics card and ask you to install the drivers.
Again, this is the typical steps you'd take, but you really need to read your user's manual to confirm. For example some boards have jumpers you need to physically change on the motherboard.
For your power supply, you want a PSU from a company like antec, enermax, seasonic, Fortron (FSB) etc. Don't get a no-name PSU, they are dangerous.
Good luck!
