sdds has provided an excellent source of information. I'll just try to clarify a couple of points:
The entry for Yellow Fever, on the NetDoctor site, reads "Certificate of vaccination may be required". This certificate is only required for peolpe travelling to Guatemala from infected areas. If you're travelling directly from the UK, you won't require this certificate.
NetDoctor is a bit vague on the risk of malaria. For more details of the areas in Guatemala where the risk is highest, see the World Health Organisation site, here:
http://www.who.int/ith/countries/listg/en/index.html
If you are entering a risk area, the WHO site recommends 'Type II' measures. This means that, as well as the usual anti-mosquito measures (e.g. nets), it is recommended that travellers take chloroquine. See here for more details on this:
http://www.who.int/ith/countries/vaccination/en/index.ht ml
Chris