Africa is a brilliant destination to volunteer on your gap year, although it's not known for having many paid work opportunities. Even teaching, the mainstay of jobs abroad, isn't well-represented, as many schools simply can't afford to pay a foreign teacher. Instead the majority of these roles go to volunteers.
You're most likely to find paid work on your gap year if you travel to popular backpacking destinations like South Africa. Here you might be able to pick up temporary bar work or similar if you ask around. Wages aren't going to be particularly high, more pocket money than anything substantial.
Otherwise you'll be hard pressed to find paid work unless you get lucky.
Africa is arguably the best continent there is for volunteer work, offering a huge array of projects that allow you to explore your interests at the same time as making a real difference.
If you're interested in humanitarian work, Africa provides opportunities to work with orphans, children stricken by poverty, disabled people and children, or generally offering support in medical facilities. There's also a huge range of volunteer teaching opportunities.
Many areas of Africa are developing rapidly, and the narrative of it being a universally poor continent is simply no longer accurate. But there is still severe poverty and disease in some regions, and volunteering there gives you the opportunity to really help people in need. It also gives you great experience to show employers when you return home.
Africa also provides incredible opportunities to volunteer with animals. You can bottle feed monkeys in Malawi, cuddle tiger kittens before they're old enough to maul you, dive deep into coral reefs as part of the conservation effort, and many, many more. After all, Africa has some of the best wildlife on the planet.
As with any volunteering project anywhere in the world you should think very carefully about your strengths, weaknesses and natural interests before committing to anything.
Whatever volunteering place you choose, it's guaranteed to be an experience of a lifetime.