BTC dropped and same as the investment itself, it's a loss this week but still we are in a good run! 
What are your thoughts this week?
Cool. A loss is always a part, that's fine because you're just getting started and as long as you aim for long-term there's not much to worry (probably). What are your thoughts this week?
What is the best gambling site to invest in and what is the minimum amount?
Great question. If you're aware of this thread, you can see that there are two gambling casino s/he invested. On top of it, there are others as well posting their investment result and simply not the same result compare to yatsan. I think the question should be what gambling site suits your need to invest with. Bustadice has a 2% (dilution) fee and Yolodice seems doing great based on OP results.
Most likely the minimum investment would be 0.001-0.01 BTC or there are none.