I'd suggest buying a shirt you like that fits you well. Do NOT wash it. Take it apart at the seams & make a pattern by tracing the parts. You may have to press the seams down on pieces first to get them to lay flat. Press, do not iron, to keep them from being disorted. I use freezer wrap paper to make patterns. You can use newspaper or any paper. If it's not wide enough then tape pieces of paper together. Draw a diagram of your shirt on a separate piece of paper. Make notes of each seam width, type of seam, i.e. overlocked, flat felled, hem type/length, etc. Also note the width & length of each piece, direction of cutting of each piece on grain, bias, etc. Buy fabric, making sure you get extra yardage for shrinkage. Wash/dry the fabric to preshrink it. Then cut out the pattern & sew it together, using the same type/size of seams as the shirt you bought. You don't have to be a master seamstress to do this. Think of it like putting a puzzle together. Each piece fits in it's own place. Once you have a pattern made & it fits you well then you can make your own from then on.
I have done this for many shirts, jeans, shorts, etc most of my life, as I'm small framed & could not find clothes, I liked, that were small enough to fit me well. Most store bought clothes had/have to be to be altered to fit me. I recently made a flannel shirt from an old one I had worn for several years. It was one of my favorite shirts & when it got holes in it I ripped it apart & made a pattern. Right now I am making summer shorts & t-tops from knits using a pattern I made years ago.