Volunteering is a great way to test out if a global health career is in you future or not. In the majority of cases, that first volunteer trip you'll have to pay for yourself. I would encourage you to do your own fundraising for the trip and not take out additional student loans whenever possible;
So, that first trip. It can be life changing or reinforce that you really like working in your home country. In my case, I volunteered while on study abroad through my university. While I was mostly doing observations, it was enough for me to realize that global health was the way I wanted to go. In particular, after observing nurses in Mexico and where they worked, I knew I wanted to study human resources development.
For other students I've had, however, the trip abroad made them realize how important it was for them to work in their own country. They realized that the problems at home were enough for them to tackle and they felt more comfortable working domestically. It was an important realization to have because they took the chance to test the waters and found where their passions lay.
Working abroad and having a global health career can sound great on paper, but until you test it out for yourself, you'll never really know what works best for you --or the people you will serve in those positions. Get yourself out there, into the world. It's worth the trip!