Buy a Spanish book and enroll in a class,
OR
If you don't like books you can always learn Spanish with interactive course. There are many courses, but I highly recommend Rocket Spanish, I've learned with it. It's not expensive and it is a full course and you can learn Spanish in 3 months or so.
See the related links below for suggested sites.
Rosetta stone
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There is no generally "best" method. The current thinking on college campuses for students learning a second language at college is that the communicative approach works best.
You can buy Rosetta Stone. It is a software and is the fastest way to learn a foreign language. Some of the languages you can learn are Spanish, English, Hindu, Mandarin, French, and Arabic. There are 30 languages in all. But it is quite expensive (at least $100!). Supervisor's Note: Rosetta Stone is actually between $300 and $500! The fastest way to learn a language is to speak it at all times. If you can't move to a country where that is the main language, then you must pretend to be there so that you only communicate in that language until you learn it. HUGE amounts of listening and reading authentic material in the target language is essential, to the sum of about 10,000 hours if you want to be good.
Most language learning programs such as Pimsleur and Rosetta Stone are inadequate not by the quality of the material but by its very limited quantity. No one will ever gain fluency with the meagre vocabulary that is learned from such courses despite their flashy advertising and exorbitant prices. Their appeal is primarily to the general laziness of Western society, who want to do everything effortlessly; when individuals who do the hard work are seen, they are either labelled "geniuses", "naturally talented", "privileged", "lucky" or "on steroids". Such an attitude is as caustic as it is false..
The fastest way to learn any language is to speak only that language. If you can't travel to a place where Italian is the main language, find some people who speak Italian and start talking. It doesn't matter if you don't know anything -- they'll help you learn to speak the same way a baby learns, by doing it.
The same approach doesn't work for everybody. Young children may learn best simply by being with a person who speaks to them in the language. Some people (adults, usually) are very analytic, and the language will make more sense to them if they read or hear explanations of the features of the grammar. Most people probably do best with a mixture--some explanation of grammar, some listening, some reading, and lots of practice speaking and writing the wordsx and phrases they have been exposed to.
The fastest way to learn any language is to speak just that language - if you cannot travel to a place where Spanish is the main language, find some people who do speak Spanish and start talking to them. It doesn't matter that you know NADA, because they will correct you and help you learn.