I agree with Niyati and Sandra. I personally took the route of getting field experience first, to make sure that the lifestyle of development work agreed with me. It's important to get experience early on living abroad in a developing country because many employers want to see that you have taken the initiative to get "field experience" and that you are aware of the sacrifices (e.g., being away from home/friends, living with the discomfort of minimal amenities, etc.) and challenges (e.g., having to communicate in different languages, being able to travel somewhere new with little notice, etc.) that come with this rewarding career.
International development offers lots of different opportunities in many different developing countries, and not every opportunity may feel or look like it's the right fit for you. Don't be discouraged if you don't think that you can brave it out in the Congo -- not everyone can! Try to find a job or pursue a volunteer opportunity in a country that is safe (pls don't underestimate safety; it is the most important thing!); has a friendly culture that you think you would enjoy or appreciate; and presents volunteer or work opportunities that will allow you to learn something new while contributing something meaningful. If you can, work directly with locals, so you get plenty of exposure to the culture. I couldn't find an international job in college, so I tried to be creative and think of an international research project that my university might be able to fund. If you're in high school, go to the career services office in your high school; counselors can have access to some great resources.
When you are in the field, it is very important to network with expatriates (or "expats") on the ground who are working in different capacities for charities, government, volunteer organizations, etc. and see what types of work you might be interested in, what type of lifestyle/pay that work comes with, what types of experience employers in that line of work are looking for. Making connections within the (usually) supportive expat communities in developing countries is one of the easiest ways to break into the field and be successful. Once you make the connections with people working on the ground, ask them questions, join their social events and keep in touch. When job opportunities come up, these people will think of you and let you know about these opportunities (and it can't hurt to reach out and ask once in a while -- in a respectful and friendly way, of course!)