This is only a technical story. It has nothing about the arty things. Hopefully you have a clear idea on what to say and by which means you can say it. You must have done some investigations on what is possible in this media (size, available colors, only short time visible, maybe partially covered, competes for attention)
In illustrator you must set
- the color model (RGB, CMYK, BW, idexed )
- the physical print size of the artboard (= to be able to give to some elements exact wanted sizes)
- raster effects rendering resolution, 300 DPI should be generous for printing on shirts
- work color space (assuming you have color management in use, a calibrated monitor, you want to see onscreen a plausible forecast of the printed result and you have got the color profile of the printing system)
- after completing your artwork: the final output file parameters due the printing system you want to use
The most important thing to do soon is to get a detailed info "How the printer wants the final file to be prepared" Without that knowledge all your efforts can vapourize just because your work is not printable.
some possible fatal errors:
- basic wrong data type (the printer wants a bitmap, you have a vector file or reversed)
- too low bitmap resolution
- too big bitmap resolution, impossibly high file size
- too complex vector drawing, impossible to plot with the plotter in reasonable time
- non-printable colors used
- not noticed that CMYK colors need a white background
- the printer wants uncolored places as transparent, you give them a file with no transparency
- vector drawing in wrong size
- the printer wants a print PDF, you give to them one made for onscreen watching
- your artwork contains text, you have made a file that can't be used without having your fonts installed (=you didn't outline the texts to curves)
Online print services give reasonably detailed info.
If you use a local printer, get the quidance in email or printed. All talks is a waste of time without a document. Insist to see the physical references "this is what has gone out and this is the file preparation receipe for it"
If the printer gets arrogant and refers only to some general "all the normal customers should know that -principles", he does not earn your money.