In Russian (as well as in many other Indoeuropean languages) in addition to the gender and number system there is a system of grammatical cases. Each noun, adjective, pronoun etc. have 6 different case forms (for each number and gender if applicable). Which one is used depends on the grammatical role of a word in a sentence.

That is a rather hard entry point for (let us say) an English-speaking beginner. You hardly can learn that from occasional questions in forum. You need at least a textbook with some theory and a set of exercises.

So the forms of "ты" (you) are the following:

Nominative - ты
Genitive - тебя
Dative - тебе
Accusative - тебя
Instrumental - тобой, тобою
Locative - тебе