1) Where To?The first thing you need to figure out is where you want to go. Different itineraries come with different price tags. Are you doing a 2 week trip to London, Paris, and Rome or are you going on a 3 month adventure across Europe?
2) Your BudgetFigure out what kind of budget you need for the trip you want to do. Do Google searches and read some travel blogs from people who have done this trip before you.
3) Get Your GearThis is the fun part and when things start to get real. Go to a travel store or one of the big box outdoor stores like MEC or REI to get what you need. Most first time travelers buy way too much stuff so read up on what other people have on their pack lists.
4) Book It!It's super easy to book your flights and hostels online yourself so don't stress over it. Online booking engines like HostelWorld or Kayak allow you to look at results from all over the world and let you choose which ones are right for you.