You need to buy a PS2 Fliptop lid kit, and PS2 boot CD/DVD pack.
I got mine from Estarland.
http://www.estarland.com/Playstation2.cat..product.22064.html
They also have Slim fliptop shells now, so you won't have to worry about the warping, probably
http://www.estarland.com/search.search.PSTFT.html
And here are the swap magic discs you will need, with either of the two above products.
http://www.estarland.com/Playstation2.cat..product.20766.html
That's it.. around $40 shipped probably and you're good to go
The lid kit is kind of flimsy, my brothers and mine were kind of warped, so you have to use some strong tape, to hold it down in place. I used electrical tape to match the color of the console, but electrical tape can stretch over time, requiring re-application.
You just start the tape on the top of the lid near the right side of the console, keeping it tight as you run it down the side, and finishing it on the "underside" of the top half of the console (not the actual bottom). That should hold it down. Electrical tape will work, packing tape might work if you use clear, or you can even try scotch tape.
The reason you need to tape it, if it becomes warped is because there is a push-down censor that the flip-top has a protrusion to always push down on - telling the console that the lid is closed. If the lid is warped, it will not be pushing it down all the way and it won't work properly.