hey 

no you can not fill application form without 12th marksheet. class 10th and 12th certifictae are also compulsory along with class 12th marksheet. you need to provide proof of your class 12th score. to eligible for neet as well as medical college you must score aggregate of 50% (45% for obc/sc/st) in class 12th with PCB/biotechnolgy as main subjects. for this you have to provide proof which can only be through class 12th marksheet. 

goodluck!