This is usually the first step in deciding on a fabric. Will it be knit, woven or non-woven mesh?
Woven fabrics are much more stable than knits, which also makes them less forgiving. The garments will need to be well fitted to be comfortable.
Some patterns will specify that they are meant for knits. This is a very important distinction. Patterns drafted for knits are drafted smaller because of the stretch. Never use a pattern drafted for knits with wovens. It will not fit properly.
Are you making leggings or a t-shirt? Then you need a knit fabric. You’ll probably want something in a medium to heavy weight for the leggings. Will the t-shirt be layered? Go for something lightweight if you’re layering and something heavier, if it will be worn on its own.
There are some fabrics that are not woven or knit, they are non-woven mesh. This includes polar fleece and felt. These are very specialized and aren't used much.