For empirical guidelines:
1st- know the common microorganisms causing the infection.
2nd- useful to know guidelines in which hospital you works
3rd-offer choice for penicillin allergic patients.
4rd- consider resistance
Basically these are the commonest a/b used. How to remember this?
Streptococci- Benzylpenicillin ( I pronounce it as streB)
Staphylococci-Flucoxicillin ( I pronounce it as staF )
anaerobes-Metronidazole ( anae for azole )
Gram -ve- gentamycin ( gen)
and usually i love to use my lecturer's simplified table

credit to my lecturer :Dr J. Q