Nouns ending with -o, -im, -om and -um are usually masculine.
o médico (m) = the [male] doctor
o fim (m) = the end
o som (m) = the sound
o atum (m) = the tuna

Nouns ending with -a, -ção, -dade and -gem are usually feminine.
a força (f) = the force
a médica (f) = the [female] doctor
a contribuição (f)  = the contribution
a caridade (f)  = the charity
a bagagem (f)  = the baggage/the luggage

Be careful with nouns ending with ão (but not ção) because they provide no absolutely sure indication of gender. There are also several exceptions where a noun ending in a is, in fact, masculine, for example:
o dia (m) = the day
o poeta (m) = the poet
o telegrama (m) = the telegram
o problema (m) = the problem