While having dirty thoughts my penis discharges itself(not completely but white thick liq) even i had not touched it .. need sol ?

You're 18yo: At 18, you certainly will have many hormonal issues, many of which heighten your arousal impulses and can cause a heightened sensitivity with premature ejaculation and even Early ejaculation without any manual stimulation (similar to a "wet dream"). If it is becoming an issue or public nuisance for you, you can see a Urologist for an evaluation and Rx medications if truly need be. Best wishes.