If you are trying to upgrade the graphics card in your desktop, once you open your side panel, the wattage of your PSU can be found on a sticker on the silver "box" that has the red, yellow and black wires running out of it. If it says 400W or higher you can go ahead and install your graphics card.

When you are buying your graphics card, make sure you get the right type. Most computers now use PCIEx16 graphic cards. Make sure you choose the right physical size. If you have a mini tower or a slimline, you might only be able to use a "half height" card. Be careful when you install a PCIE16 card, it has a "tail" that locks it in. Some people flex the card to insert it. If you do, you can break the traces. Your card mfgr will NOT warrant a card damaged in this way.

when they say 400 watt minimum, they mean that MINIMUM, you should really consider a 500 or higher watt PSU if it recommends 400. The number is the CAPABILITY of the unit, not what it uts out all the time. Your motherboard will draw what it needs.
You did not say what make and model computer you have. You need to match the PSU to the form factor (for example ATX) and you need to make sure it can physically fit in the inside of the case. People who bought "slimline designs" are NOT going to fit a 500 watt PSU in their case.

Other than that, the PSU can be installed in 10 minutes with just a screwdriver. It might be a VERY good idea to get help with this because. I do NOT mean offense, your question is one that would tend to make us worry about doing yourself. I am just trying to keep you safe and save you money from buying the wrong stuff, and from blowing up your computer. This is NOT covered under your warranty.

Good luck!

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