I'm visiting my girlfriend in Thailand, what do I need for tourist visa?

I’ve been in an online relationship with a loving lady from Thailand.

I met her through A Foreign Affair and we’re in a healthy relationship despite not meeting in person.

We have video calls every now and then, and corresponds regularly;

and because it’s the holidays… I’ve decided to visit and spend the New Year with her.

I’m very excited! There’s roughly few days left and it’s a sudden decision,

so I’d like to ask for advice on getting a tourist visa. I’m a U.S citizen and

I’d like to know what are the requirements I’ll need in order to get a fast approval.

But I’ve also heard that there’s no need for a visa if I’m only staying for less than a month…

is this true? I really need some helpful input!