Which medicine should I take for instant period.

No such thing. But plan B helps. Thats how it works to stop a pregnancy.
If you have time. to plan, such as for a specific event (vacation or wedding) controlling your cycle with birth control pills is probably the best option. Taking progesterone is often used to induce menses, but not instantly...menses usually occurs after a 10 day course of progesterone. Some herbs--parsley/ginger-- are said to stimulate uterine contraction as are mango/papaya and having sex. Unreliable, though:)