Make sure that you use a power supply that has the correct FORM FACTOR for
your case and motherboard. There are at least 7 different FORM FACTORS
(PC/XT, AT, Baby AT, LPX, ATX/NLX, SFX, and WTX). The most widely used for
today's computers are ATX, AT, and Baby AT.
Choosing the right FORM FACTOR is very important. For example, an incorrect
power supply may not even be able to plug in to your motherboard, and/or may
not be able to fit your case. Not only that, but the wattage rating for some
power supplies is insufficient for the power demands of many computers.
Since you do not know what a CPU is (Central Processing Unit that mounts on
your motherboard, the "brains"of your computer), I assume that you have
limited technical knowledge. Therefore I suggest that you ask one of your
guru friends to help, or take your system to the technical support people at
a local computer shop and ask them for assistance. Don't just jump in and do
the replacement by yourself. You could end up with a ruined motherboard and
a ruined CPU.
For further reading on the subject, click on
http://computer.howstuffworks.com/power-supply2.htm