New trades should focus on majors pairs because at the initial stage of trading they don't require having knowledge of trading subtleties like liquidity, spreads, execution. Also their movements are driven by some very common factors and that's why easier to analyse (although not to issue correct predictions)
That depends highly on you. I think there is no simple suggestion or clear guide for those, which currency pair to choose. Each currency pair has its own specific behavior and movement, as well as its own nuances, which are recognized and understood by traders with sufficient experience of trading in the market. In fact, focusing on trading currencies that a trader knows well enough, you can get a significant advantage in trading, so choose currencies that you really understand.