There are 4 important Portuguese contractions that you need to know. But take just a brief moment to appreciate what a learner of our fine language needs to absorb: we'll, she'll, it'll, isn't hasn't wasn't wouldn't, shouldn't, and on and on. I bring this up because Portuguese contractions are an object of so much complaining -- it's helpful to realize that they're (another contraction) not the only ones that do this. That said, the contractions used in Portuguese are actually very intuitive and easy to pronounce.
Common Portuguese Contractions
Brazilians use these in everyday speech:
no & na
» in, on, at + the : in the office; on the table; at your house.
em + o = no
em + a = na
Notice that the no & na are expressing: in the. For the longest tie (~a year) it would really confuse me saying no -- it's jus such a powerful word in our language and hard to attach a different meaning to it!
Of course, there are PLURAL versions also. But let's keep it simple. You can get by just fine without the plurals until you are already making your own sentences. In most cases, you just add an "s" to get the plural anyway so no big deal! In this case, we have nos & nas.
num & numa
» in, on, at + a : in a mood; on a cloud; at a party
em + um = num
em + uma = numa
do & da
» of + the : of his; of that; of the
de + o = do
de + a = da
ao & à
» to, at + the
a + o = ao
a + a = à