As far as what to do first, the T5 swap is going to be a challenge for you. That said, the other people who've responded to your post have pointed out that there is a ton of info on this swap so you won't go into it blind.
What are your priorities? Is the C4 you have on its last leg? Is there something else that NEEDS replacing?
As I wrote before, I will always prefer a stick over an auto but there are some other things to consider to. Do you have a dual bowl master cylinder? I believe that is an essential upgrade. It is also relatively easy to do and it won't take you long, unlike the T5 swap. Furthermore, it is downright cheap compared to the T5. This project will force you to learn the art of 'brake bleeding' which is an essential skill.
I was amazed at how much of an improvement in drivability the rack & pinion gave me. With all of the mods I've made, that was the most noticeable in terms of drivability. There are non-power rack kits that won't break the bank and they are relatively easy to install with normal hand tools.
There is an old saying: if it ain't broke, don't fix it.
If your C4 is working fine, you might want to focus on smaller projects first so you can get your feet wet. I don't know you, but if you get frustrated easily, that is more reason to take small bites of the apple first. If you dive in and tackle a big project which is beyond your skill level, you might sour on the whole car!
Here is another old saying: getting there is half the fun.
I know patience is not something high on your list of character attributes to obtain (it certainly wasn't on mine at 17), but don't be in a rush to get everything done at once. Tackle a small project, then drive the car for a few weeks. After that, you'll be ready for the next project. My Dad is working on a 1952 Ford Sedan Delivery and he has not driven it in 8 years. The car is not in a drivable state because he tried to take on too many projects at once. Some guys on this Forum have the patience to do a rotisserie restoration and they are okay with waiting years before they can drive it again. I am not one of them. My point is that if you keep the projects small, you'll be able to drive the car more, and thus be able to ENJOY your hard work during the process.
No matter what mod you do, there will always be someone with a faster car than you. Try to avoid equating an awesome car with a fast car. I have a friend with the ugliest Fox body you'll ever see. It is also the fastest car I've ever been in. The interior is even worse than the exterior. I can't imagine him impressing a date with that car. I guess he is fortunate he is already married then, huh? My point is that you might want to focus on the things that make the car enjoyable to own and drive, rather than just making it fast.
Alright, that is enough preaching for one night.
