I'm willing to invest into crytocurrencies but not sure how to start ones I'm ready to dive in. Shall I go through Bittrex, Coinbase or find a legit seller here in BTCtalk? What would be your advise?
It depends on which payment system you prefer to use.
Some exchanges accept credit cards, others paypal while the majority accept only bank wires (and thus you need to wait several days...)
In my opinion the true spirit of cryptocoins is going peer-to-peer. So find a reliable user and arrange a deal with him!