Whats the difference between a sentra GXE and an SE-R and what mods can I add to the sentra to make it like an SE-r

Different engine, dude.
If your talking about a 1991-94 GXE, then you'd have to go thru the hassle of swapping an SR20 into it.
If you're talking about a 2002+, then I don't even know if it's possible yet to swap a QR25DE from an SE-R into a 1.8 GXE.

The SE-R's are really almost a totally different car.