Take a piece of fabric (whatever you want, silk,cotton or whatever). The dimensions are again according who you want it (short,large,long,slim).For a large and long one take 1m*1,5m.
Fold the 1m in two.
And then fold the other side too.
Cut the neckline as you want it.You will cut the corner that is totally folded,not the open one. You can cut a simple one for now and later adjust it as you want (open back,one shoulder out,asymmetric).
Then draw your sleeve and side line as you want it (very slim sleeve etc)
Then leave 1cm following the line you drew and stitch each side exactly at the line.
Me i put pins at the micrography. You will stitch it at the machine.
After turn it the other way and...
Your new t-shirt is ready. Once it is ready you can change it as you want.
Another tip is to use the zig zag scissors to make it look better at the neck,sleeves and bottom.
I know the demonstration i did for you looks silly like a barbie t-shirt but it is better before cutting your real dimensions to try it in a small piece.