How can i teach my dog to sit?

i cant teach my dog to sit...i try makin it sit then i gave it food..i told it to sit and i push his down then i gave him food..i do that repeatly while saying sit the whole time but didnt work..it just stare at me ..it startin to learn but i dont know how long itll take...everytime i say sit it thought i was goin to give it food..