1st, welcome to the site!
2nd, NEVER talk about street racing or you will get thrown off. None of the reputable message boards tollerate this. Be sure to say "I am going to the track.....". This is a huge liability issue and street racing is HIGHLY illegal and bad for all of us. I pay higher insurabce rates because of guys who do this and it really ticks me off PLUS I want to live and my kids want to live.
3rd, your question was like asking "so I just graduated from med school and I want to perform a brain transplant, what do i do next". I am NO WAY busting on you and we all love a new guy. Without guys like you scooping up stangs, this whole thing goes away. But, there is so much to discuss to answer this question.
4th, I will try to give you some advice:
A 12 second NA car is pretty hard to achive, without something major. You can do this with heads and cams (among other things), but you most likely need a power adder. Nitrous alone, will get you to the mid 12's EASY AND for less then $600.
What are your goals? If you want to go straight with LOTS of power for cheap, NO2 is your best bet. But, read up on it. You have to do it right.
There is always the bolt-on route, but they will not get you to 12's alone....perhaps low 13's. I speak of underdrive pullies, throttle bodies, upper plenums, cold air, performance chips, exhaust systems etc... and of course, 4.10's.
4.10's are a great mod, but you need to be a good driver. Oh, did I tell you that the best mod is practice???? However, if you plan a blower, then you want 3.73 max most likely.
If you want a supercharger or turbo, you have to change a lot more on the car. Things like ignition, timing, plugs, and the entire fuel system. Not to mention a custom dyno tune. Figure on a $4,000 - $8,000 expense depending on who does the work (remember what I said about nitrous).
Also, more power means you need to get it to the ground. So better suspension and a stronger rear end. It really takes about $10 - $12K to build a GT into a monster.
The best thing you can do is READ, READ, and READ. Ask lots of questions and be sure what is best for you. Don't listed to me, listen to all of us. Take it all in. Go to other sites like
www.modulardepot.com. Some of the fastest Mustangs in the world are on that site. Also, don't jump at anything too fast.
One way to get into the 12's very quickly...tomorrow actually is to trade your car in on a 2003+ Cobra. Sometimes we spend lots of money building a car, when this often is the best option.
Look at my mods. I am running, or should be, mid 13's. An X Pipe would help me out quite a bit and heads and cams should get me into the high 12's. I would think this gives you some idea.
Hope this helps!