first stang, any ideas for first mods THANKS

Just bought an 04 gt and best i can tell it has aftermarket exhaust and everything else is stock this will be a daily driver and see some track time since i am only about 20 miles to drag strip any help or ideas would be appreciated have looked on the web and bought a few mags the choices are overwhelming  

Hey, congratulations on the purchase.  Of course opinions fly everywhere, and here are mine.  
Depending on how deep your pockets are I would suggest getting more air into the engine.  I'd probably start with an aftermarket plenum and TB.  Looking around you can get the trick flow pieces from summit for $250 for both parts.  I'd then get an aftermarket air intake.  All three of those parts will set you pack about $500 if you can swing it all at one time.
Something else to think about is that you should probably spend a few dollars making sure the car is running in top condition.  Make sure the oil was swapped (probably to synthetic), have the plugs swapped.  I'm assuming it's a 5-spd car, so get a feel for the clutch and make sure it's good to go, but if it's any auto, I'd get the fluid changed.
After the air is taken care of, I'd probably add some gears to the rear end to make it accelerate quicker.  As a daily driver I wouldn't go with 4.10s.  3.73s will wake it up and not completely kill gas mileage.  With those air modifications you should not only see an increase in power, but also fuel econmoy unless you put your foot in it more.
So now that you have the air intake, TB, plenum, gears, and a fresh tuneup, I'd either go an electric water pump, like one from Mezeire, a 255lph fuel pump, headers and X-pipe.  I put Kooks headers/X-pipe on all my mustang, but they cost a lot.  It'll run probably $1400 for the headers/X-pipe with cats, and probably $500 or more to install.  That's why I'd save those until last.  If you chose to do the headers or not I'd get a tune next.  With all the extra air your are pumping into your car you'd definately benefit from an SCT dyno tune.  My 01 Cobra picked up 22.3whp and 18.6lb-ft or torque with just a tune and the car only had a magnaflow cat-back.  It was the best $450 I spent on the car.  Not only did it give me more power, but the throttle response improved, and highway driving mileage went up.
I like things to match, in finish and brand if possible, so here is a further reccomendation;
I'd get the trick flow TB and plenum (75mm) and trick flow air intake.  Summit sells the TB and plenum as I stated above for $250, and the air intake was $260 I think.  
For the tune up stuff, I'd swap the oil for Royal purple, change the plugs (whether they look good or not), change the trans fuild if an automatic, top off the brake fuild, check the coolant level, and swap the windshield wipers.  If you are going to go to the parts store you might as well pick those things up.
I wouldn't replace the antifrezze unless if really needs it because I'd get the mezeire electric water pump next, along with a new accessory drive belt.
At that point the car will be WELL taken care of and all the regular maintenance items will be good to go.
For the gears, I'd run FMS 3.73 gears.  With that you should have the fluid swapped to a good quality synthetic oil, and since you are going to part time race it, get the aluminium rear diff.  They cost about $175.
At this point I'd get an SCT tune, and go play!  I'd forgoe the headers and x-pipe because of the cost and minimal power gain.  It probably run about $1500-$2000 (if running the Kooks) to get the headers/X-pipe and install.  For that price I'd get a nitrous kit, another tune, and probably be in the 12s at the track with 100hp jets.
If you want to continue the modifications process, before I do the headers, I'd still get a set of partiot stage II heads, a mid-level set of cams, springs, and when getting the heads replaced, I'd do the headers so you save on labor.

Good luck!  I hope this wasn't too out of line with what you wanted.  

I just bought a 1999 mustang GT and I put a BBK cold air induction kit, BBK 70mm throttle body and flow master mufflers and noticed a decent amount of power increase.  Also when accelerating you can hear the cold air induction hissing (sounds pretty sweet).  I also plan on putting steeda underdrive pulleys, sct tuner, and 3.73 gears, which i hear are all good first mods.  Now that your going faster you need to be able to stop faster.  I put cross drilled and vented rotors with ceramic pads and that is a great upgrade.  

Hey, I have to agree with Nosjunke, If you look at my profile I did almost exactly what he said to my Mustang,The cold air intake 75mm TB and plentum a set of underdrives with the zex wet nos set up I got 439.39 rear wheel torque on the Dyno I run 12.3 @ 121 in the quarter  

be careful thinking about mounting cros drilled rotors, I had a set and they cracked around the holes.  stick to vented rotors or 2 pc drilled.

all the other options are good mods.