I would use a facing of some sort. Rolling the hem under has never worked for me(but I'm of intermediate experience). I have used bias tape and ribbed knit in the past. Instead of folding the hem over, stitch knit strip or bias tape(opened up)to neckhole with right sides together. Trim seam allowance. Fold tape over to the inside of neckhole and "stitch in the ditch" on the outside along the seam line. Trim excess facing close to stitching.
Here's a site I found with different techniques...

Here is a link that might be useful: stretch necklines

Knits like t-shirts are hard to sew without stablizers. Get yourself some tear away stablizer. It will hold the neckline stable to sew it. It makes it so it won't stretch out of shape. Cut your neckline and iron on a stablizer strip so it's on each side of your stitching line--about an inch strip will do.

Take your neck band and sew it together. If you are making it much larger you will have to use new banding material or it won't fit around the opening without stretching it out of shape. If you can't find the right color use a contrasting one. OR-- you can use the existing one like bias tape so it won't matter if it's stretched.

Put the seam at center back on the wrong side and pin or baste, then sew on the band. Tear away the stablizer. Turn it to the right side and under 1/4 inch and baste by hand on the right side over the stitching of the side of the band already sewn. Then machine sew it.

Thankyou so much for your info. I would need pictures for this tho, as I am a very beginner.

I don't know what type of sewing machine you are using but a lot of machines have some pretty specialty decorative stitches that you might be able to just take off the old band and stitch around the neckline. You would need to use a stabilizer on the back as dowlinggram suggests. You might also try adding some type of trim such as a pre-gathered lace ruffle or braid or picot, check out the trim section for ideas. The main thing is to use stabilizer to keep the shape.

Thankyou for your post, Reds2k, I will look into that. And thanks for the other reply's, don't hesistate to offer any suggestions in the future.

Knits won't ravel so you could just cut the neckline a little smaller than you want it, use a stretch overcast stitch to stabilize the edge of the fabric, fold under and then top stitch it as narrow as you like...using a stretch stitch that looks like a straight stitch would be best as you need to maintain the stretch to get it over your head.