There are very few openings for ad-hoc volunteer work in Africa. Unless you've got some expertise, and are prepared to stay for at least a year, you're unlikely to be much use anyway. What Africa needs is people with skills. Just 'wanting to help' isn't enough. In fact, your presence may be disruptive for local staff and management, prevent locals from gaining employment or cause a drain on resources.
For formal volunteer work, which must be arranged in your home country, organisations such as Voluntary Service Overseas (VSO; in the UK) and the Peace Corps (in the US) have programs throughout Africa where people, usually with genuine training (eg teachers, health workers, environmentalists), do two-year stints. Similar schemes for 'gap-year' students (between school and university) tend to be for shorter periods, and focus on community-building projects, teaching or scientific research. Almost all these projects require an additional financial donation, which may be raised by sponsorship and fundraising in your home country.
The following international organisations are good places to start gathering information on volunteering, although they won’t necessarily always have projects on the go in Africa.
African Impact (www.africanimpact.com)
African Volunteer Network (www.african-volunteer.net)
Australian Volunteers International (www.australianvolunteers.com)
Coordinating Committee for International Voluntary Service (ccivs.org)
Frontier Conservation Expeditions (www.frontier.ac.uk)
International Citizen Service (ICS; www.volunteerics.org)
International Volunteer Programs Association (www.volunteerinternational.org)
Peace Corps (www.peacecorps.gov)
Step Together Volunteering (www.step-together.org.uk)
UN Volunteers (www.unv.org)
Volunteer Abroad (www.goabroad.com/volunteer-abroad)
Volunteer Service Abroad (www.vsa.org.nz)
Worldwide Experience (www.worldwideexperience.com)
It's hard for outsiders to find work in most African countries, as high unemployment means a huge number of local people chase every job vacancy. You will also need a work permit, and these are usually hard to get as priority is rightly given to qualified locals over travellers. You're unlikely to see many jobs advertised, so the best way to find out about them is by asking around among the expatriate community.
Most opportunities are usually in the fields of aid, conservation and tourism (such as working in a lodge or hotel, as a tour guide, as a diving instructor...); the latter sector is the one most likely to be looking for skilled overseas workers at shorter notice.
If you’d like to spend more time in Africa than a vacation allows, experiencing what it’s like to live and make a difference to the local community, it’s all possible by Teaching English as a Foreign Language (TEFL) with i-to-i TEFL. No previous teaching experience or local language knowledge is necessary. Get your free i-to-i TEFL e-book now to find out more about TEFL, what TEFL jobs are available and how it can get you exploring the globe...