Start with the grammar basics. You can choose to sign up for a class and benefit from the classroom environment to learn your basics and start speaking Spanish, or you can chose to go down the self-taught route. For the latter, you will need to arm yourself with a good basic Spanish grammar book. Set aside time from your weekly schedule in order to read it, study the grammar, do the practice exercises and correct yourself.
Read a lot. It might sound silly if your goal is to learn to speak basic Spanish, but this language has one major advantage: It is pronounced just the way it is written. So once you have your Spanish ABC's down, you will be able to correctly read and pronounce most texts in Spanish, even if you can't understand everything you read. This is why you should arm yourself with another good book: a Spanish-English dictionary.
Immerse yourself in the language. The only way you will practice Spanish orally is if you are spoken to in Spanish; you will have to respond. So find and exploit all opportunities you have to speak Spanish: with a pen-pal, at your local Mexican restaurant, in Spanish class, through a language exchange with a native Spanish speaker, and so on. Improve your listening comprehension skills by watching movies in Spanish (subtitles in English if you have to, subtitles in Spanish if you are brave), listening to Spanish radios (online if you have to), reading the Spanish language newspaper or even watching Spanish TV channels.
Use Spanish constantly. It's the only way to keep the speaking skills you achieve, and even improving them in the future. Practice. If you feel confident enough, go back and repeat all the steps but with more difficult grammar books and by setting new challenges for yourself: Learn 10 new words in Spanish every day, read at least 10 news articles in Spanish every month, or have at least one full conversation in Spanish every week. Keep Spanish in your life and you will see that from speaking Spanish 101 to the next level is not that far.