When you first create your character in Stardew Valley you get to choose what you want your farmer to look like, but it's also possible to alter this appearance if you change your mind later on.
In order to change your appearance in Stardew Valley, head to the Wizard's Tower. You will need to have at least 4 hearts of friendship with the Wizard if you want to change your appearance, so work towards that if you don't already have it. It is pretty easy to get friendship with the Wizard since he loves the following three items that are readily available in the Mines: Purple Mushroom, Solar Essence, and Void Essence. You can also buy these items from Krobus in the Sewers if you don't want to to head down in the Mines right now. Anyway, give him gifts twice a week until you get enough friendship levels.
Once you have 4 friendship hearts with the Wizard, enter his basement below the Wizard's Tower. You will see a big, golden shrine here called The Shrine of Illusions. You can make an offering of 500g to this shrine to change your appearance.