One of the words for you in Russian is ты, which is declined like this:

Ты, of course, is the singular form of ‘you’ that is used when talking to someone you don't have to be more formal and polite with. You can use it when talking to a child. You can use it when talking to a pet. You can use it when you are talking to a brother or a sister. You can use it when talking to one person whom you know well. So, if you suddenly spot someone walking down the street, and you know that person very well, you might say:

In contrast to ты the word вы is used to talk to a single individual when you are speaking more formally. Mastering when to use ты and вы is a bit problematic for American English speakers for several reasons. The obvious reason is that there is only one word for you in English. Previously the general rule for non-Russian speakers of Russian was that one should use вы until a particular Russian suggests otherwise to you. That's not the worst rule on the planet. But nowadays the transition from вы to ты is taking place much more rapidly than previous generations could have dreamed of. Nonetheless, I am still advising my students to start with вы with anyone older than them, and then allow the Russians to suggest making the change when it suits them. If you are a college age or highschool age American talking to a Russian who is the same age as you, go for it and use ты right away.

One last comment. You may have noticed that the instrumental case has two different forms. The more common one is тобой, so that should be your first choice in speech. But if you are ever writing poetry in Russian and need to have an extra syllable in the line, feel free to use тобою.