Newbie here and I want to start trading soon. Just want to get tips on when to invest on btc and eth. thanks in advance! 
Welcome to Bitcoin/Cryptocurrency world buddy. If you do really like to make some trades between these coins then as a begginer or complete newbie you should know first important fundamentals to consider first before tending to jump in into trading because its not really easy on making money, you should really be knowledgeable to make profitable trades.Talking on when to invest on btc and eth is when the price is on its dip or lowest because once price do goes up you can make profits on it when you decide to sell.Thanks buddy! What exchanges can you suggest to me? I plan to invest on 2 or 3 exchanges. Is Poloniex and Bittrex really the good choices?
poloniex and bittrex is a good choices for you but maybe you need to familiar first with only one exchanges so you don't confuse to trading with 2 exchanges. and if you want the other exchanges, you can try with cryptopia and coinexchange, but they are only for small coins which is not listed in the big exchanges so be careful with those 2 last exchanges, cryptopia and coinexchange.