Well there's a couple of problems you're going to face. First is that your computer is an old HP. Many HP models don't have AGP ( a special slot for a video card) and many HP models don't like third party cards.
Also, to replace the video card, you will have to open up the computer. Computers can be dangerous (electric shock, sharp edges) and you also risk damaging the computer. If you've never worked on a PC before, I would recommend you have your parents help you or take it to a PC repair shop.
Can you look on your computer for any model number or model information, like "presario 2700" or "deskpro EN", it may be on the sides, front or back of the computer? If you can find out what model it is, I may be able to look up what type of video card it can use.
If you decide you do want to install the card yourself, make sure you unplug it first! I was younger than you when I started playing with the insides of computers and actually damaged my 286, so be careful.
I know the reason why is that i need a video card but i dont know where to get one or what kind to get????? Please help me. 
