Although Brazil is surrounded by Spanish-speaking countries, the language here is Portuguese. Your Spanish will help a lot, though. Most Cariocas understand it, if spoken slowly. Anyone dealing directly with tourists will understand, and try hard to speak English. French and Italian are the other two languages that are likely to be at least understood.
We realize Portuguese is not a first choice as a second language, and nobody expects you to become a pro. Learning to read the words may be a useful skill, though. It will surely come handy when you have to say the name of a street, or some place you are going to.
The only letter in the Portuguese alphabet you may not be familiar with is the "ç", a consonant that always sounds like an s. The sound of the "r" is also a little tricky. It usually sounds pretty much like the h in loch, or the German Nacht. When surrounded by vowels it sounds like the Italian prego, or Corleone, thus Rio (HEEoo), and Carioca (ca-ree-O-ca). The h has a mute sound, unless combined with a c, l or n, as in chato (SHA-too), alho (A-lyoo), and punho (POO-nyo). The s may sound like an s in sábado (SA-ba-doo), a z in asa (A-za), or an sh before some consonants, or at the end of words so it's gatos (GA-toosh), and afasta (a-FASH-ta).
Vowels may have an open or a closed sound, and sometimes there is an accent to indicate. The one that looks like a little hat means a closed sound, thus você (vou-SAY) and até (a-TE). The "~" lends a nasal sound to the a in maçã (ma-SUN). The ão combination is very common in words like chão (sh-AWn) and pão (PAWn). This is the maybe the hardest sound for the foreign speaker to pronounce. To practice, imagine you are repeating a mantra that goes AWn, AWn, AWn... Here are the key sounds:
Learning some key phrases in Portuguese will surely help you break the ice. Most commercial city guides have a list of useful sentences. You may take a couple of lessons at a good language school, buy a phrase book... or click the button below and jump to Michael C. Martin's "The travlang Company" to practice online Portuguese for travelers.