The first time I went travelling, I was 19, heading to New York, Germany and the UK for a month with my friend. It was amazing, but two things no-one told me was:
1. You need less stuff than you think
2. Make sure both your hands are free during transit
I learned the hard way that you buy loads of things to wear, tipping you over the scales on baggage allowances and causing HUGE problems in subterranean train stations. I would like to now thank everyone who stopped in London and New York to help me with my stupidly heavy suitcase up and down stairs. I also learned that carry-on baggage should be as compact as possible, allowing you to reach things like passports, boarding passes, plastic bags with fluids and laptops easily.
These days when I travel, I use a small backpack for cabin baggage, and my travel motto for luggage is never to carry anything that you can’t lift yourself.