Do i need 50 thieving to get into the chest?

I believe it's 53.

So how do i make 11 thieving levels =(

Guards are a simple but slightly annoying way to get to 55. Just either use Varrock castle (with a few hundred lobs) or Ardougne if you want to steal cakes for food. You could try Fremenniks at level 45, but involves a lot of travel and also the "talk to Fremennik" option makes them a bit hard to thieve fast. They can be faster exp then guards though overall

 

 

 

Straight from the Tip.It Thieving Guide.