Namaste is correct but not universal. Which greeting fits depends on who you are speaking to.
| Who | Say |
|---|---|
| Anyone, any time | नमस्ते namaste |
| More formal, an elder | नमस्कार namaskaar |
| A Muslim friend | सलाम वालेकुम salaam waalekum |
| A Sikh | सत श्री अकाल sat shri akaal |
| A close friend | क्या हाल है? kya haal hai? |
| On the phone | हैलो hello |
A greeting on its own is awkward. Add आप कैसे हैं? (aap kaise hain?) to a man, or आप कैसी हैं? (aap kaisi hain?) to a woman.
Yes — the greeting changes with the gender of the person you are addressing. That is grammar in Hindi, not extra politeness, and getting it wrong sounds as odd as "he are going" does in English.
Reading an answer is one thing; being corrected while you say it is another. EkGuru tutors teach one to one from $6 a lesson, and most offer a trial.