The Vietnamese alphabet is Latin-based, and therefore mostly recognizable to English speakers, but it has major differences. You will have to learn the diacritic marks, which indicate pronunciation. Go to the Vietnamese section of 101Languages.net to see a copy of the Vietnamese alphabet and how to pronounce it. The site also has an explanation of the six tones used in Vietnamese and how to recognize and pronounce the various diacritical marks.
Start using Vietnamese quickly by learning basic vocabulary and useful phrases. The website Learning Vietnamese has a long list of conversational phrases you can learn and begin using immediately, from greetings to goodbyes to "Happy New Year!" The pronunciation of the words and phrases is written in phonetic English as well as Vietnamese, so you can start speaking the language even before you have mastered the intricacies of the Vietnamese alphabet.
More extensive information can be found at Northern Illinois University's SEAsite for Vietnamese. Meant to accompany the textbook "Spoken Vietnamese for Beginners," the site nevertheless contains material useful for anyone wanting to learn Vietnamese quickly. Work through the 20 lessons, as well as the accompanying online quizzes, interactive exercises, and vocabulary flash cards. You will find some grammar lessons, but mostly the emphasis is on immediate application. The lessons are divided into themes such as "Shopping and House Rental," "Daily Activities" and "At the Restaurant."
You will always learn a language more quickly if you begin to use it. If your town has a Vietnamese population, do not hesitate to try out some of your skills with your neighbors. If you're shy or have no Vietnamese-speaking neighbors, join a language exchange. MyLanguageExchange.com has an extensive list of students who would like to practice English in exchange for letting you practice your Vietnamese. Language exchanges usually take place via email or chat. If you would prefer to remain a bit more anonymous, practice your language skills through message boards. There are several threads at the Asia Finest discussion forums concerning Vietnamese.