Hi, I'm new here.
If you had to recommend a beginner where to invest btc, what would you say?
invest in a gambling site greatly accelerate your bitcoin into a double.

There are only two things you could possibly mean. Either gambling, which is not an investment by a long shot, or investing into a casino's bankroll, which will defenitely not double your investment...
Both are the risky place to invest your money as there is no guarantee that it will give you profits in long run, best investment is to invest in bitcoins and hold some of them and rest use in trading to grow them.