If you want to become fluent quickly in a foreign language, it is hard to go past Intensive Language Learning courses. Whether you are a complete beginner, or already have some background, several weeks or months (depending on your time and budget) of immersion is the most effective way of gaining fluency.
Ideally, this course will take you overseas to a location where you will be surrounded by your language all the time. Think of what a valuable investment of your resources this is! You are immersed in a foreign culture and language, learning and growing, and doing something completely different from normal. This is change with a capital 'C'. I can guarantee that you will return home with a new appreciation of your flexibility and ability to rise to the occasion, and that you will not be the same person who left.
Intensive Language Courses are available in many different countries and cities. If cost is a factor, consider South American schools, but if you have always dreamed of visiting Europe this may be a fairly economical way of seeing at least part of it in some depth.
There are several different approaches to intensive language learning, but if you are starting out from scratch you will make infinitely more progress in an environment that helps you learn grammar, rather than the more casual 'get out on the street and pick it up' approach. The latter approach is fine for short-term, superficial acquisition, but if you are trying to build a foundation for future learning a more structured approach is better. Some cultures do not enjoy hearing their language butchered by foreigners and see it as a lack of respect - in which they are right.
An intensive language course can be very stressful since you are trying to learn a great deal in a short time, and probably studying harder than you have for many years. However, the very intensity of the experience almost guarantees a measurable degree of progress. Most courses involve six to eight hours of class time, practice and homework five or six days a week, which can leave you totally exhausted. If you are starting from the very beginning, a month of this will equip you to deal with basic conversation and everyday matters, but you will probably still have difficulty following the news, or entering into significant conversation, or taking care of detailed business. You will have the pieces of the jigsaw, but have difficulty putting them together. On the other hand, you will be a month closer to that 'Aha,' moment, when you are following a normal paced conversation on a variety of topics, and not mentally translating everything back into English.
The most effective living situation during your course is with a local family. Although this means that you will not be able to retreat into solitude as easily, it also means that you are surrounded by the language and culture, and you have more chances to practice and absorb the language, as well as an opportunity to develop relationships which may last well beyond your initial visit. Most schools can arrange this for you if you ask, and it is well worth the added stress.
Unless your goal is merely to be able to party in a foreign language enrolling in an Intensive language course is well worth the investment, and your hard work will result in measurable progress, as well as great satisfaction. You may even return and repeat the experience at a higher level in the future.