To learn or not to learn

Half of the world speaks English. However, the number of native speakers is rather small to be compared with the Earth population. Meantime, there is the need in English everywhere. Those for whom English is a foreign language have serious problems to get into it.
The point I am going to make is what could be the ways of an efficient learning and acquisition to avoid wasting time, energy and money to become an effective user from the very begging of studying.
Are that English Grammar or extended vocabulary, or some frequent phrases and word combinations to concentrate much of attention while learning, or perhaps the phonetic must be a key to understand spoken English, pronounce correctly and remember surely the language material one is sitting on?