site collection content db ldf file consumes huge space in C drive

I am having a site collection and it has dedicated content db. We have uploaded documents and now the size has become huge. when I checked the ldf file in sql server location am seeing the size is 18GB +. Can anyone help how to delete content of this db and free up the C drive space. i tried to shrink this content through sql management console. but the next day again its filled up with space and now my c drive is having some 30 kbytes of memory only. Mine is a dev machine with SP 2013 and sql server 2012 is installed