i would do a lot of research and figure out what u want to do with the car b4 spending money..if u plan on motor swap, find out as much info as possible..ex: what intake will fit with ur swap, header, exhaust..etc..i just installed the skunk2 IM and i have a CAI for my GSR, well with the new IM the intake seat really high and doesn't fit well..i found out i should get ITR intake since the IM sit so high..so those are the kind of thing u'll run into..so now i have to customize my intake if i want it to look good, or spend more money getting the ITR intake..just make sure u do research..
as for basic things, i would start with maintenance first..check fuel filter and replace..spark plugs..O2 sensor..valve cover gasket, oil pan gaskets..replace struts/shocks and or springs if u want to drop it..replace rotors and pads..check brake fluid, if it's dirty then flush and refill with new fluid....check tires and pressure...check A/C make sure it's cold enough..summer is coming..i would start with all that first b4 modding..