do u always have to iron a 100% cotton shirt?

Only if it has a lot of wrinkles. They almost always don't have too many if you put them in the dryer. It also depends on what you're going to be doing, and if you're wearing something over it like a hoodie, there's no site in spending the time ironing it.

I only recently ordered a 100% cotton shirt offline and i don't have the time or patience to turn it for a larger size. How do I keep the shrinking of the t-shirt to a minimum?

Always break it on the 'delicate' cycle in COLD water and let it 'hover dry' ... the heat in the water and the heat in the dryer can both 'cause shrinkage' in 100% cotton. But you fundamental to 'check the label' to find out if you can tell if what you are getting is 'genuine cotton' that will shrink or 'pre-shrunk' cotton, which can be washed in warm/hot tone down and dried in the dryer.

I Recall that the dryer actually shrinks it, but how long should i wash a cotton shirt, that has never been washed before, in hot water? Its 90% cotton and 10% polyester
How sustained is one cycle?

One series then dry the **** out of it