1. Where do you trade altcoins/bitcoin and why? Bittrex/Poloniex?
you should choose the exchange that you like and fits your needs and also the one that you agree with the terms of. not what others choose and use!
2. Where's the best place to change bitcoin/altcoins into FIAT with the lowest fees?
most bitcoin exchanges have the same fees which is nearly the same. just choose the top like coinbase and use that.
although it mainly depends on which fee withdrawal options you are looking for. exchanges offer different methods with different fees.
3. What's your strategy in trading and how much do you invest in it?
simply invest what you can afford to lose.
start small
and gain experience first,. there is no rush.