Obviously antibiotic usage varies from place to place, and from time to time, but this is a brief guide.
Staphylococcus --- flucloxacillin
Cellulitis (strep and staph) --- penicillin V and flucloxacillin
Allergic to penicillin, use erythromycin
Anaerobes --- Metronidazole
Gram negatives --- Gentamicin
Broad spectrum --- Cephalosporins or augmentin
Urinary tract infection (UTI) --- trimethoprim
Fungal eg. Candida --- Clotrimazole
Herpes virus --- Acyclovir