My advice: at the beginning, choose one, maximum two options that are easy to remember

Saludar de manera universal – all of the most common Spanish greetings 

1.  ¡Hola!- hello or hi is the most common Spanish greeting and can be used at all times
2. Buenos días- good morning;
   this expression is more formal than the simple hola, but  also used in informal contexts
3. Buenas tardes- good afternoon;
   it’s commonly used between noon and sunset, this expression tends to be more formal than simple ¡Hola!
4. Buenas noches- good evening and good night;
   This is because in Spanish, there’s no word for evening. This means that, you can greet someone by saying Buenas noches but you can also wish them a good night in the same way
5.  ¿Cómo estás?- How are you?
   very popular expression, it can be used any time.
6. ¿Qué tal? –How are you?;
   a slightly informal but equally popular version of the phrase:  Cómo estás

Saludar más coloquialmente – colloquial phrases

1. ¿Qué pasa? –literally means: What’s happening and is equivalent to the English What’s up
2. ¿Cómo va?/ ¿Cómo te va?/ ¿ Qué tal te va? – another commonly used Spanish expression that translates to How is it going? How are you?  like ¿Qué tal?
3. ¡Hombre, + name – very often used expression combined with the phrases: ¿ Qué tal? ¿Cómo te va?, e.g.:  ¡Hombre, James ¿ Qué tal? ¿Cómo te va?  as: hello/ hi bro, how are you?
4. ¿Qué hay? – What’s up?

 Responder a un saludo de manera universal- universal forms of response to greetings

1. Muy bien, ¿y tu? – I’m fine, how about you?
2. Estupendamente, ¿y tu? – I’m great, how about you?

Responder a un saludo más coloquialmente- :

1. Tirando, ¿y tu? – I’m ok, how about you?
2. Sin novedad , ¿y tu? – Nothing’s changed, how about you?

Despedidas- How to say bye

1. Hasta luego – So long, see you later
2. Hasta la próxima – Until the next/ up next
3. Hasta pronto – So long, see you later
4. Adiós – Goodbye