Every time I turn the key to start the motor it blows the 30 amp fuse what is the cause and how do I fix it

This may be due to a bad starter, i would need to know what year make and model car this is and after the fuse blows the engine wont try to start or what does the fuse control and if you just turn the key on without going all the way to crank the engine does the fuse blow. if the answer is the fuse wont blow unless you go all the way to crank postion then you have a bad starter, good luck and let me know if theres a problem.

If the fuse is blowing when you turn key to start position then this is where to look for the fault ,between the ignition switch and the relay then the other side of the relay to the starter motor solonoid