Design My Own Logo

I know someone [barley] who has started her own clothing business making shirts dresses tanks bathing suites tracksuits cup cozies etc! They are all her own design and logos I want to do this very badly but I cant find what it is she uses i want to design the basic shirt/dress etc i want then i want it made so I can sell it how do i do this? what do i use? what will the costs be?

thanks!