Keep in mind that this game is very much a trial and error game. All of the best players are the ones that spent the most time trying to figure out how to do all the crimes. The quicker you can get to the most difficult crimes and crack them, the quicker you will raise in level and gain money.
Also, make sure to horde as many treasures as you can get, ranking is dependent on lots of different things but treasure is one of the big ones. With that said, don't stress it too much, rank will come with time. Have fun with the game and don't be afraid to ask questions when you're stumped.
Also, the helpdesk is a great place for tips and advice when you are just starting out- http://www.booty-master.com/help.php