Which power supply do I need to support my system?
Solution 1:
Your question is a bit short on details (exact CPU model, number of RAM sticks, additional components etc), but assuming you are using integrated graphics or a low power discrete GPU, a 300W supply should be fine.
You can find an online PSU calculator here, where you can enter all all your system specifics. It is good to plan ahead for any extra hardware you may add (extra HDDs, graphics card, etc). The calculated will give you a minimum and recommended wattage for a PSU (go for at least the recommended amount).