Like I have said in the comment above that Rome wasn't built in a day! You can't just throw away seeds in the fields and expect to have rich crop, you have to till the soil properly, clear out weeds, level it, add manure etc etc before sowing seeds, even after that you have to take proper care of your crops! Similarly just chanting mantra won't help you much, there are other things that needs to be followed religiously before chanting mantras directly.
Mind is just like a child, you have to cajole it, pamper it, talk to it, listen to what it is saying before making it listen to what you want to say. Basically you have to de-condition your mind. Mind is similar to other organs of the body, it's not something that is against you, in fact it is a very potent instrument of your body once you get to know how to deal with it!
The first step to get rid of sexual/lustful thoughts would be to stay away from such activities for few days to start with. When I say activities I mean reading/viewing/discussing/visualizing such material. Gradually increase the duration, take small steps initially. This will help in boosting your will power. This is the first step of
Ashtanga (eight limbs of yoga) referred to as yama (abstinences). Strictly follow this routine for few weeks.
Now as far as chanting a mantra is concerned it should not be a parrot like act, it will not lead you anywhere! You have to chant the mantra with full concentration, mentally focusing on each and every word of the mantra while chanting it. But concentration can not be built over night, it's a long process. You first need to follow a routine for at-least 21 days before it becomes a habit and then move on to the next step. Just chant the mantra loudly (not too loud!) in a peaceful & secluded place with full concentration, start with 5-10 minutes daily and gradually increase the duration. Don't rush! After few day start chanting it mentally for mental mantra japa is highly recommended. You can read about it more here.
Hope this will help you in your spiritual journey!