Aap, tum, tu: three ways to say you
English has one word for you. Hindi has three, and picking the wrong one is the fastest way to sound either cold or disrespectful. The good news is that one of them is a safe default, and you can get through an entire first meeting using only that.
The short answer
Use आप with anyone older than you, anyone you just met, and every one of his relatives until someone tells you otherwise. Nobody has ever been offended by too much respect.
All three, side by side
तू is not a beginner tool. Between two lifelong friends it is warmth. Aimed at a stranger it is a provocation. The line between the two is not something you can judge from outside the family yet.
The verb moves with the pronoun
This is the part people miss. The pronoun and the verb ending travel together, so learn them as one block rather than swapping the pronoun into a sentence you already know.
The other pronouns
Hindi does not split he and she in the pronoun. यह and वह cover both. The gender shows up in the verb and the adjectives instead, which is why agreement matters so much in this language.
Saying it about yourself
One rule to carry
When in doubt, आप. Then let the family move you to तुम when they are ready. That invitation usually arrives as a joke about being too formal, and accepting it is its own small milestone.