Sheets are completely flat so ironing them is probably the easiest ironing you'll do. They give you great practice at smoothing out two or more layers of fabric at a time and this would help you with other, more difficult, items.
Of course, this doesn't take into account fitted sheets, which it's almost impossible to keep flat because of the elastic pulling the corner together.

Fitted sheets cause problems solely because of the shape of the corners. Start by matching two corners together and folding one over the other, so you have a corner with two layers of fabric

Change hands to pick up the iron while holding the stretched sheet tight. Get the iron and place it on the sheet, replacing the other hand and keeping the sheet tight, then iron that corner.

Do the opposite side and then fold the sheet down to finish.