"What's the main difference between actually riding a road bike and a normal bike?"
The position of the rider on the bike. A drop bar road bike puts the rider in a more efficient position. Touring/sport bikes tend to be a bit more upright in the rider position while racing bike get the rider's torso and head lower.
"How hard is it to pick up riding a road bike?"
For an 18-year old? Piece of cake. At 18 I 'trained' for a couple of weeks and then rode a 210-mile 2-day ride as easily as I put on a two day bender. You're 18. You can do anything! Find a local club and ride with them. You'll learn the ropes in no time. If you're in good health and in just 'decent' condition for your age, you will soon be causing old men to gasp and strain to hold your wheel.
The physical aspects of road cycling will come to you rapidly. The mental aspects and finesse that smooth a rider's rough edges will take a few seasons of wearing out tires and your legs! My advice is to study the guys you will be training with and riding with. Figure out what they do well and learn from their strengths and from their weaknesses.
"What do I need to start from scratch with cycling (Road bike cycling)? (And how much does it cost roughly?)"
You'll need a bike, helmet, shorts and shoes. Also some spares for the bike. Starting out, a good used bike from Craigslist might set you back $400-$1000 and about the same for shoes, clothing, spare tires/tubes, patch kit, pump/CO2 inflator, etc.
The cost can be spread out over time and upgrades and changes can be made as your conditioning and experience builds. Your cycling goals may also be a moving target.
"What's the best brands and most reliable?"
Well, that's where you're going to be in far better shape than those of us that started cycling 40 years ago when much of the equipment was trash. Today, almost everything above the department store level bikes is decent to very good.
Knowing your budget and initial goals would help the folks here better point out what might work best for you.
