First of all, how are you going to Cuba legally? I hope you're aware of the restrictions against Americans.
Assuming you know what you're doing, then you also need to see what kind of shots you need. You better do that ASAP - sometimes you have to take a series of shots that take awhile, or with malaria pills you have to start two weeks before you leave. Do some research to find which medical centers near you specialize in travel inocculations, then call them and tell them which countries you are visiting - they'll tell you what you need.
Make multiple COLOR copies of your passport along with a copy of your airline ticket or eticket itinerary (on the same page) and leave a few copies with your family or friends, and take a few with you (but store them in different, safe places - don't put them all with your passport, purse, or wallet). That way if you lose your passport or it gets stolen, you have a backup that will help you replace yours. The ticket or itinerary proves that you have a flight out - which many countries require to see before entering their country. Also take a few small, passport-sized photos with you - that helps the process if you have to replace your passport while in a foreign country.
As far as packing, do some research on the weather, the country customs (do they expect you to dress nice, or are casual backpacker clothes ok?). Not every country is as casual in dress as North Americans. Be aware of the TSA guidelines for what you can or can't take on the plane, and check everything else. In your carry-on, you can't take any liquids or gels more than 3oz, and they all must fit in a single, quart-sized ziploc baggie; and no sharp objects.
There's so much more, do your research on the country and keep asking questions. Good luck!
15 years of international travel