You need means, motivation and opportunity when taking up any new activity, such as learning a new martial art, dancing, pottery or …
First you need to have the means. You need money, and most importantly time. For some, money is a barrier; and for some others with significant work, other/family commitments, finding the time is the limiting factor.
Next you need the opportunity. You need to find an instructor whose class you want to attend and who is prepared to take you on. You both have to want to work together, although he is the boss, you have to like him too. Now a days most schools are turned into businesses, but you can still find instructors (rarely) who are involved for the love of the game.
Finally you need motivation. Aha, motivation is highly important as it is the main thing that will keep you going once the novelty wears off.
In today’s world there are a number of things you could be doing with your money and your time. I personally find martial arts appealing and commendable. If you study the philosophy behind the art, it will gradually change your character (for the better I hope :-).