Just like you said buy btc when it's on low period. The sure way to increase your btc with that amount of money is by trading in trusted exchange depend on your country. But to be able to make stable profit trading, you need to have the knowledge and you seem don't have that kind of experiences yet. So it's better for you to expand your knowledge first and start small.
Secondly the most safest way maybe by lending it in poloniex or any trusted website. Don't rush about what you do. It's easier to lose your money than make it double if you didn't have what it takes.