I would not attempt to do this if I were you (or if I were me for that matter). No you cannot simply replace the graphics card.
If you want experience building computers you should start with a desktop because they're cheaper and easier to put together. Laptops are designed from the ground up to be as integrated as possible. This is a nearly impossible project for someone with tons of pc knowledge and it doesn't sound like you've even put a PC together before. If you go ahead with this you probably wont end up with a working product.
Also, if you attempt to build a desktop and run into issues, people will be much more willing to help you and will be more useful. If you noticed, no one else has commented on this post. That's because it's not an original question. This, or a slight variation, gets asked often and people are tired of explaining to others that they shouldn't try this.