Your best starting point is to have either a pressing need, or a powerful desire to master the language. This may seem obvious, but interest combined with passion brings the best results. If you're not really enthusiastic about learning the language, then you will end up stalling or quitting. Your sustained interest is decisive - which is why courses with a teacher often work better than teaching yourself, as the structured setting and company can carry you over the inevitable slumps. If the choice of language isn't made for you, pick a language that you can use or practice often in your daily routine, or create such opportunities for yourself. Join a speaking club where the enthusiasm is infectious, or spend some time with cultural things that motivate you. Rod Ellis in "The Study of Second Language Acquisition" (1994) states "SLA research ...views motivation as a key factor in L2 learning." If possible, find materials like books, magazines, websites, radio, TV or movies in the target language as a supplement to other education. Like-minded buddies (in real life or on chat-sites, for instance) are even better. The more exposure you have to the language in actual use - whether passive (listening/viewing) or active (conversation, reading) - the better.