Just to add to what Beppie already said:
You asked about what might be to blame for your irregular periods:
In general, the menstrual cycle can be influenced by plenty of things: for starters, it can take up to 5 years until it gets regular, it can be influenced by nutrition or lack thereof (think eating disorders), medications, life changes or hormonal imbalances and disorders (i.e. polycystic ovarian syndrome).
Hormonal Imbalances can - in some women.- indeed later on cause problems with fertility. However, try not to worry that much about it yet. Even if you end up being diagnosed with a disorder or disease that is associated with fertility problems, fear not, there are plenty of new treatments around that assist women in getting pregnant once you want to get pregnant.
Also, if you get such a diagnosis, make sure that if and when you get sexually active, you still use birth control methods and practise Safer Sex religiously. Especially women with PCOS often get pregnant while on the pill. But don't worry too much about these things first, take it one step at a time and see what's behind your far apart periods.
In any way, like Beppie said, a doc (and at that, a gynecologist is indeed your best bet) can evaluate what's behind your problem.
Before you head to the doc, check out WebMD: Topic Overview: Irregular Periods, it has a useful section on preparing your doc's visit to make sure you don't forget to tell her/him everything you wanted to tell about your periods.
"Through repetition the magic will be forced to rise."