"something that plugs into the USB and convert it into wireless"
(Just to explain: I seriously doubt anything that does that even exists. If it did, routers would be obsolete, and nobody would buy them or make them.)
I've never heard of such a magical device ever even existing, so..
Get a router. I'd recommend a Linksys WRT54GL.
This is where I got mine from: http://www.ncix.com/products/index.php?sku=17408