Congratulations with graduation, but long-distance moving is really a shock. I am pretty sure that you will try to put everything into your bag, but your back will suffer from it in the future. Trust me, nobody needs four hoodies for summer vacation in Barcelona. So we have to make our bags under ten kilograms of weight to enjoy the view.
The first thing which you have to do is to create a color palette and stick to it. Blacks, greys, navy look absolutely fine together, but you will not look like a walking uniform. Take that neon graphic t-shirt, but it will need at least three pairs of pants to match. Capsule wardrobe may sound boring, but at least it helps to be a civilized traveler.
Merino wool is really an expensive thing to have, but it is almost like a magic when travelling. It does not smell even after hiking as many as you wish. Cotton, on the other hand, is pretty heavy and retains moisture. Spend some money on a couple of quality woolen t-shirts, and you will be able to wash them wherever you want.
Folding the clothes in suitcase is quite a standard thing to do, but you should try compression cubes. It squeezes everything in your suitcase and provides some extra space for unnecessary souvenirs. Once I managed to pack two-week outfit in 40 liters Osprey bag thanks to it. It felt like cheating, and it was cheating.
Do not try to take things just in case, because this is a mistake of inexperienced travelers. You will be able to buy anything you need somewhere abroad. It is much worse to travel with a heavy backpack than missing a flight. Just try to keep it light and simple.