the order of doing things varies and depends upon loads of things (not least of all, the driver and the car).
also, track cars and road cars are different.
some might disagree (and it does depend upon the car) but the order of doing things with a dedicated track car might be:
[- safety gear (cage, seat, harness, extinguisher, etc)]
- lightening
- tyres (biggest difference to lap times and, *bonus*, simplest mod)
- brakes.. down, may be
- suspension.. up, may be
- engine/transmission - lsd, etc
stripping the interior (lightening) from a road car isn't for everyone and cages/seats/harnesses/track tyres aren't ideal either, so you're left with the age-old three (brakes, suspension, powertrain).
this is where things become complicated as, with a track car, the last thing on the cost-effectiveness-list is engine mods, but they're much more appealing in a road car.
personally, i'd still start with the brakes and suspension as improving those will increase your confidence(*) in the car before you make it any faster.
(*) depending upon your attitude and experience, you may well find that some form of advanced training (on or off track) supercedes all of these suggestions. you're the driver.