If you have a young seedling the first two leaves to come up will be the cotyledodones they look like small oval leaves, above them will be you first two true tomatoe leaves.
Before potting up this young seedling, pinch the two cotyledons off. Now as the plant grows there will be established leaves (aka branches growing out below the top two leaves, leave them on the plant. A sucker is a leaf that grows in betwen the established branches and the main stalk. These should be pinched off with the thumb and forefinger.
Is that an adequate description?