First, you need do human activity such as ingest nutrient enriched bread product. While consuming said item, while mouth cavity is largely clear you use tooth stones and mouth muscle to engage another human with guttural mouth noises. If they are pleasing, other human will engage. Continue this process until best friend received.

Friendship is, fundamentally, liking somebody and they like you and you enjoy spending time together, supporting and communicating with each other.

Importantly:

you don’t get to be in any sort of genuine relationship with someone else without their consent and mutual interest.

If you like them and they do not like you, that’s the end of it.

As for how it begins, it drives on every situation: maybe you saw someone play pool well and you’d like to know how the game works from their perspective.

Maybe you had a significant conversation with an acquaintance of ten years that drew the two of you closer together.

Maybe you saw someone’s jacket and thought “what a cool jacket.”. The initial interest can come from anywhere.

Zuckerberg?