Take a slightly oversized shirt, if you wear a small get a medium . Cut the seam on each side, but do not cut the it up to the sleeves.
After that has been done insert small metal grommets on each side of the now cut seam. An inch apart on each side. Then find a ribbon you like (you can use the excess fabric from the cut t-shirt if you'd like).
Lace the ribbon up the side of the shirt , similar to lacing a shoe or a corset.
Then cut the neck line of the t-shirt. Make sure you cut it wide enough that your shoulders are exposed. But also make sure that its not cut to wide. I have ruined many of my shirts by cutting the collar to wide and then it falls down and I look like a silly billy. Anyway I would guesstimate cutting about three inches from the neck line, to the sleeve hem.
Enough so your bra straps are visible, and it goes only slightly off the shoulder.
The next step is the sleeves. I like to gather mine into an off shoulder cap sleeve. But you can either leave them as they are. Tie a small strop of ribbon, or of the t-shirt fabric around the front and back base of each sleeve(for the off the shoulder cap sleeve.) Or you could cut the sleeves into a spaghetti strap, or cut the thick sleeve into a two finger width off the shoulder strap.
I would pair the shirt with a short jersey skirt, or with black pencil jeans. Of course with Converses.
Keep what you wear simple. Yes clothes are an expression of your individual self. But also remember that your individual self is completely unique already and doesn't need to be dressed up and painted like an Easter egg.
I hope this helps.