I find that the biggest factor with tshirts is the stabilizer. It was recommended to me to use two sheets of cut away, and I resisted this for a long time. When I finally did this, I had a lot less problems. As far as needles are concerned, I find that just using a new one is more important than the type for tshirts. Good luck.

I find that knits, such as t-shirts, stitch with a nicer finished design, and hold up through repeated washings without puckering and curling when stablilized in the following manner. (1) Iron on a small piece of fusible stabilizer to the back of the fabric where the design will be stitched, careful when ironing not to stretch the knit fabric. (2) Hoop a piece of light/medium weight cut-a-way stablilizer. (3) Lightly spray the fusible with K2000 fabric adhesive, then align and press gently to position on the hooped cut-a-way. (4) Place a piece of wss on top to keep the stitches from sinking into the knit fabric. After stitching the design, I remove the excess wss from the top, then trim the cut-away to 1/8 inch around the edge of the design inside the t-shirt. I have tried many different ways stitching t-shirts, and this method has proven to be the best. I do not like hoopburn on t-shirts, so I prefer to not hoop, and, too, it is so easy to stretch knit fabric when hooping.
Hope this helps.
Angel

Thanks for asking this question because i'm battling with t-shirts myself and I have 2 t-shirts I want to do before Monday but are to scared because its not mine. I find that when the design have a outline it isnt always done neatly.

Make sure you use a ball point needle also because the sharp one can cut the fabric rather than slide through.

I have tried embroidering tshirts but have given up, never happy with them, so Angel's comments give me more hope and I will try again.
thank you.

I second the ball point needle answer. Good answer from anangel-good luck!

I tried the two sheets of cutaway and it worked thank you for the useful tips!!

Ball point needle and stabilizer. I've found that a fusible cut away, or fusible leave-in stabilizer works very well. I think I have half a billion yards or so of t-shirt fabric, so have been practicing lots. That has been my solution combination. A topper layer of a WSS or iron away stabilizer really helps if your embroidery foot tends to "gouge" the fabric and pull it out of shape. Good luck!!