Most clothing patterns come with sizing options. You'll see lines to cut for different sizes...like a "range" of large, medium, small. Patterns are fairly true to size based on your measurements.
Where things get tricky the first few times of sewing patterns are things like collars, back yolks, button fronts...anything requiring interfacing and two fabric layers. You'll be cutting out shapes that make no sense until the final assembly. Make sure you understand "wrong side" and "right side" of your fabric.
Sippers are fun, as are button holes.
When you are done, you'll wonder why shirts cost $30 and not $500, with all the labor that goes into them.
The more you do, the faster you'll get. I've sewn hundreds of theatrical costumes and after a while, you don't bother reading pattern instructions....everything just falls into place.
Hint....get a rotary cutter and big mat, and straight edge.