Count your correct and incorrect answers against the answer key. This applies the current JEE Main (Paper 1) marking — +4 correct, −1incorrect (now including Section B numericals), 0 unattempted — for your marks out of 300.
JEE Main reports a normalized percentile, not raw marks — this tool gives your marks out of 300, not a percentile or rank. How marking and percentile work: JEE Main exam pattern & marking.