A trip like that is definitely doable! If this is the first time you're travelling alone, it's probably a good idea to figure out and book where you're going, how you're getting there, and where you're staying before you leave. This makes it less stressful for you, because you won't have to worry about ending up with no where to stay for the night, and it's a lot less stressful for your parents/guardians, because you can leave a detailed itinerary with them and they'll know where you are and how you're travelling in case something happens.
While you should definitely go where you want, you might want to think about the fact that Scandinavia is pretty much the most expensive area in Europe, which over the course of a month is going to add up. I'm not saying don't do it, because I love Scandinavia, just keep it in mind. It does have the plus of being a really safe place to travel though, and it's a fantastic area.
In general, the first step is probably to figure out your budget and time frame, because these are the biggest restricting factors.
Then you should do some poking around and figure out what exactly you want to see and do while you're away. While you're doing this, try to determine roughly how much time you want to spend at each location. I like to use an Excel spreadsheet to track all the relevant information, so you have it all at your finger tips. You will almost certainly find that you want to spend more time travelling than you actually have, so choose your must dos, then work in the other things. For example, if you really want to visit Oslo and Helsinki, and Lapland and Copenhagen are both "want but not desperate", Copenhagen would make more sense because it'll involve less travel time. That's not to say you shouldn't visit Lapland, but try to avoid huge detours if you don't have your heart set on it.
The next step is basically a continuation of the previous step, but once you know where you want to go you should lay out your trip in a sensible way. I like open-jaw plane tickets, that is flying into one city and out of another, and in general you should avoid any doubling back when at all possible. Again, Iceland to Oslo to Stockholm to Copenhagen to Helsinki makes more sense than Copenhagen to Helsinki to Oslo to Iceland to Stockholm. For this step I like to use google maps and rome2rio.com is also really helpful for figuring out how to travel between places. Make sure you keep in mind any time specific activities when planning this step!
Once you know where you're going, you can figure out things like plane tickets (although you should keep them in mind when planning too!), hostels (I like hostelworld.com, but there are other sites like it), and the other similar stuff.
Happy planning!!!
(also: are you doing the IB!?!!?)
