your cycle begins on the day your period starts. Normal, healthy cycles can last anywhere between 23 and 35 days for different women. The "28 days" often mentioned are just an average.
Whats normal for you depends on what it has been like for you before: if you're just starting to menstruate, it's perfectly normal that you cycles are still irregular.
It can take up to five years until your cycles are more regular.
For more, check out On the Rag
Spike: (In response to being asked to fight a troll) "I would, but I'm paralyzed with not caring very much."