You don't say how old you are - or how long ago you started your periods - so this is just a shot in the dark. Sometimes, whe you first start having periods, they are realy irregular. If they've always been this way, then I think it's something you should discuss with your doctor. There could be a hormonal imbalance or issue that needs treatment.
If you've been sexually active, you may be pregnant - you can take a pregnancy test at home or get a free one at a Pregnancy Center - look one up in your area at this site:
However, with the brief information you posted, my first and strongest inclination is to tell you to see a good doctor. You can even call the pregnancy center near you and ask them about a good OB/GYN dr. you could see.
