Im planning to stay in my girlfriend's house for 30 days.Do I need an L or S1 or S2 ?

Also Im confused about the invitation letter.

Am I supposed to write the date im going to be there. And the date im coming back.
But how can I do that without first buying the tickets.

And how can I buy the tickets, without first having the Visa.

But If I don't get the Visa I will loose the money I spent on the tickets

Im confused about all this.