I would stick to Illustrator first as i think it's an easier package to learn and easier to print low cost t shirts from, i.e. vinyl cut rather than screen printing as you need a fair bit more money for screen.
Photoshop truly has endless possibilities and a plethora of options to do things with. Although the majority of people I know that use Photoshop are self taught they aren't utilizing nearly half the available options and tricks it has to offer. My best advice to you would be to pick up a Photoshop for Dummies book at Amazon.com depending on which version you have. The book really spells things out for you in plain english step by step. You'll be a pro in no time. This was a great guide for me.
Adobe Classroom in a Book for Photoshop and Illustrator. They are rather expensive, around $45 each. Both come with a cdrom full of files that accompany the tutorials. These books are second to none... seriously blowing the "dummies" series out the water. These are books I was sat down in front of when first learning the programs back in college ten years ago.
Before you start setting up artwork in either application just think about how you're going to finally print your designs. I currently print using vinyl so i can only really use illustrator for that as you need cutter guides etc. if you're screen printing then i guess you could use either app.
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