Recommended Vaccines: Yellow fever vaccination is required. The CDC recommends that all travelers older than nine months be given a vaccine 10 days before travel. The CDC website can help you find an authorized yellow fever vaccination clinic in the United States. You should also, of course, get up to date MMR, DPT, poliovirus, and varicella vaccines. Here again, hepatitis A vaccination is strongly recommended, as is one for typhoid, particularly if you will be going to rural areas or staying in private (not commercial) accommodations. And hepatitis B is recommended because the prevalence of chronic HBV infection is high in Liberia.
Malaria: You will need to take one of the following antimalarial drugs: atovaquone/proguanil, doxycycline, or mefloquine (primaquine in special circumstances and only after G6PD testing). Chloroquine is not effective in Liberia and should not be taken to prevent malaria in this region.
Other Issues: Dengue fever is another mosquito-borne illness that is common to Africa and other tropical and subtropical places. There is no vaccine to prevent it, nor medicine to cure it; acetaminophen is the usual treatment. You can help ward it off by being vigilant in preventing mosquito bites via clothing, chemicals, mosquito netting, and staying indoors at dusk.