The CDC Health Information for West Africa is the gold standard for vaccinations you may want, as well as other health considerations.
The only required vaccination (as of 2/2013) is Yellow Fever — you need your WHO card indicating that you have been vaccinated against Yellow Fever in order to enter the country. The Yellow Fever vaccine can only be administered by certain doctors and health centers, often county Boards of Health and specialist Infectious Disease doctors. You should plan on getting this vaccine well in advance of your trip as you may need proof of vaccination to get a Guinean visa.
You will need to take anti-malarial medicine while you are there, and for 2 weeks prior (at least). There are a few standard options that are prescribed for Guinea travels (Larium/mephloquine, malarone, and doxycycline), and your doctor can counsel you on the best option for you.
Many travelers take a vitamin C or immune supplement (grapefruit extract, “airborne” or vitamin C drink mixes) in order to boost their immune system while in Guinea.
It is also good to get checked out for any bugs you might have picked up upon your return.
One traveler says: “As far as immunizations, we got yellow fever (required), hepatitis A, and my daughter and I needed tetanus and a polio booster. We took Lariam for malaria preventative, one week before we went, while we were there and for four weeks afterwards. My husband and I had varying side effects from that drug. I really didn’t sleep well until I was finished with it and he had some degree of anxiety attacks and shortness of breath. We found out, after the fact, that these are some of the known side effects.”
Another writes: “I went to Ohio State travel health clinic and they recommended the following: hepatitis A, meningococcal, polio buster, tetanus booster, yellow fever, prescription from malarone and I took the live oral vivotif typhoid tablets. They told me not to worry about hepatitis B.”