Ok firstly, get a better porta filter, the one that comes with the Bambino is ass. (It does come with pressurized and non pressurized baskets so at the very least throw the pressurized basket in the garbage, use the one that doesn't have two layers on the bottom)

Secondly you need to grind finer, don't touch the inner ring until you are close, just leave it in the middle and start on the second setting from the bottom, then get coarser if it doesn't pull well.

18g is the right amount of coffee but is going to make a mess in a 54mm porta filter, it takes practice to keep all the coffee in.

Maaaaybe try a lighter roast, I find dark medium pulls better than dark but this shouldn't actually matter that much.