I'd say CorelDraw above anything else. You should get it anyway and start learning how to use it now. If you're going to stay in the business you need to know your way around CorelDraw. Probably 90%+ of what I do is in Corel and less than 10% Photoshop and I knew a lot more about Photoshop than I did about Corel when I came into the business.
Not only, as ClutchInc said, does it make your graphics sharper - because it's vector based, but in the long run it's were a lot of your income will come from. Get yourself a plotter and you can turn simple 1/2 colour jobs around while the client waits at the counter if you have to.
Then get yourself a Versacam and use that for your full colour jobs on coloured garments. Save the transfers for white only (light transfer).
So in my opinion all you need is Corel & Photoshop. It is a good idea to have Illustrator on hand as even tho' you can import eps & ai files directly into Corel, I like to sometimes check that it has imported properly by checking the original in Illustrator also.
This isn't as important now as Corel 6+ is much better at maintaining fidelity when importing ai/eps/psd files than it used to be.
Anyway, that's my 2 pence worth.
I wish you well.