First, the older you get, the harder it becomes to learn a language. So begin now, regardless of your age.

Second, you really must have a strong interest in the language, culture, or people of the target language (or at least be able to grow that interest over time). Preferably, the sound of the language should also appeal to you on a certain level. Learning a language is a commitment. That said, it is possible to dabble in different languages. It is one of the best ways to learn whether a language will interest you over the longer term (and a great vignette into another culture).

A good book on the subject is How to Learn Any Language by Barry Farber. He has a system of techniques that he's developed over the years, having supposedly learned a couple of dozen languages. It is helpful to study other polyglots, such as Sir Richard Burton, as well.

I learned French on my own. I can speak passably and understand it well (spoken and reading).

Here's my method:

1. Get a good grammar book. You can get a simple grammar book to start with. I like the Dover Essential grammars (e.g., Essential French Grammar). They're short, concise, and less likely to overwhelm. Plus, they're a handy reference later on.

2. Get a good dictionary. Start with a bilingual dictionary. Eventually, you can get a dictionary in your target language (a dictionary that natives use). Make sure it has pronunciation in IPA. You can then read the definitions in the target language, which helps add to the volume of consumed text and meaning. This is a tip I got from http://www.antimoon.com, a great language learning site.

3. Develop a set of flash cards. These days, I always use SRS (spaced repetition software). I started with SuperMemo, one of the originators of the technique. Sadly, this app is a bit neglected and the interface is poor. My SRS app of choice these days is Anki. It's a great app written by an Australian who wanted to learn Japanese while living in Japan, so it supports kana and other character sets well. Plus, it's written in Python and OSS. It works on many platforms.

4. By a Pimsleur audio course. This is probably the best way to get some speaking ability early on, with good pronunciation. Pimsleur also uses the SRS technique. There are other audio courses, but Pimsleur is the best, IMO.

5. Begin listening to native speakers. If you don't live in a country that speaks the language, you can rent movies and turn on the subtitles (turn them off later, either after you've learned a bit or if you've seen the movie a couple of times and know the story). You can also listen to the radio. Find a good station that has web broadcasts with speakers and no music (I listen to rfi.fr). Listen. Daily. Even when I barely knew the language I would put this on in the background and just let my subconscious be immersed in it. Eventually, I started to be able to pick out words. Later, I understood phrases, etc. Eventually, you'll hear something over and over and be compelled to look it up. This is how you learn the daily idioms.

6. Join a speaking group (e.g., via meetup). The only way to become fluent is to use what you've learned, even if you stumble early on (and you will).

7. Ideally, move to a country that speaks the language. Even if only for a few months. I moved to France for two years. While you're there, find excuses to get out every day and talk with people. Take a class on your favorite subject (fencing, basket weaving, etc.) with locals. Go to the market.

When learning a language, you go through stages. The first important stage is beginning to understand the spoken language. The next step is being able to follow a conversation. At a certain point, things will "click" and you'll have learned enough to where you're not studying so much as consuming. Eventually, you learn new vocabulary through use--just as with your native language.

Language is a uniquely human trait. Our brains are wired for it. We learn through saturation. Follow the advice of Antimoon and expose your brain to as much content as you can. Read or listen every day. Eventually your brain starts to work things out.

Finally, once you've learned your language, keep it. You must review or you'll begin losing it. I listen to the radio.