Question about Travel to the US (Munich)

Hello fellow IN members,

i have a quick question, after travelling the world America is on my list now.
I wanted to kindly ask you what to consider for actually being able to enter the country. I did the ESTA-registration. Is there anything else i need to consider?
I will jsut be visiting friends there. Do i need a letter of invitation or something?

I am looking forward to your answers,

A