where should i invest and why ?
It all depends on what it is you want to make money off of, how active you want to be with your investment, and what kinds of risks you're willing to take to get various types of returns. I also don't know why you're asking a public board for investment advice but whatever, I'll try to contribute.
Active investments:
Bitcoin trading - Trading Bitcoin back and forth on exchanges to make a profit off of whatever movements occur. Decent amount of risk with low returns short-term but good long-term trends.
Altcoin trading - Trading various altcoins back on forth on exchanges to make a profit (in Bitcoin). Medium risk with high reward, great short-term trends but very timing dependent, might get left holding a bag on a worthless token.
Starting a business - Starting your own business. Medium risk high reward, assuming that your business takes off.
Passive investments:
Loaning - Loaning Bitcoin to other people. High risk low reward, lots of defaults and will likely not be paid back. Skip, in my opinion.
Investment in a gambling site - Investing Bitcoin into gambling websites for a return. Low risk (most of the time) low reward, better for large investments.
I'm not telling you how to use your money, I'm giving you potential ideas and personal perceptions. This is anecdotal and in no way guaranteed to see happen.