The game is simple: when people (yer friends for example) visit your hotel, they stay for 24 hours and pay you a fee. You can then spend this money to enhance yer building. All you need to do is bring people here ( http://www.yerhotel.com/Berlin/Dry-Guest ). Oh... and you, as a register user, can visit many more hotels every day (and hope those guys will visit back :) ). So visit other people's hotels, leave them nice comments - they'll come to you!
This was a very short version of what you can read in the manual.