Your log file is growing because all the transactions made on that sql database are note commited into the main file (db.mdf).
As a first thing to do to trouble shoot this i recommend that you make a full backup of the database and that will commit all the transaction to the main file.
Then, you have several ways too keep under control the size of your log file (ldf). One is to schedule a regular backup of your log file. Another is to change the database model to simple instead of full, and that will keep commiting the completed transactions to the main file.
Addionally you need to shrink the log file to reduce it size.
You can find all this procedures in the sql help assistant.
If you have any doubt ask here again :)
Ariel