Hi,
If you're going to get a t-shirt printing company to make the t-shirts for you then speak to them first and see what formats they accept and what the specs for the design should be. Work back from there and figure out what software can produce those type of files. Odds are that they'll be happy with Illustrator files and may accept PDFs files, but check that before you do anything!
If you're printing them at home, for example using your home printer and some of that iron-on stuff then you can use whatever you like even (shudder) Word or something like that - if you're happy with the result and it prints on your printer then it's job done.
What you find easy/hard to learn is a pretty personal thing so I'd suggest you get a few free trials of popular software and see what you get on with. If you don't get on with Illustrator then maybe take a look at Affinity Designer, Pixelmator, Inkscape.
I can't stress enough though, if you're having the t-shirts printed commercially then run this past your printing company first.
Good luck.