You need to know what happens during menstrual cycles to understand why it's irregular...the Pituitary Gland in your brain starts releasing the Follicle Stimulating Hormone (FSH), when high enough this tells your ovaries to start producing eggs. As eggs grow they produce the hormone Estrogen, when this is high enough the Pituitary Gland stops releasing FSH and starts releasing Luteinizing Hormone (LH) instead, when high enough this causes ovulation where the egg breaks from the ovary. The area where the egg broke from in the ovary starts releasing the hormone Progesterone, along with Estrogen this causes your uterus walls to plump up, if you don't get pregnant the Progesterone stops being released causing your period.
It's complicated! But that's exactly the point, everything has to be working together, otherwise not enough hormones is being released to trigger off the release of hormone elsewhere in your reproductive system. It doesn't mean there's anything wrong, it just means that it takes time for everything to start working together so don't expect your periods to be regular until your body has finished developing.
For now keep track of your periods on a calendar, also note changes to discharge and things you may notice before periods like cramps, cramps can be avoided but for now these help you predict periods. Carry what you need with you so you're ready for your period, or try menstrual cups which can be used before periods, you can leave them in for 12 horus so you don't have to carry spares and you can avoid mess if your period is unexpected. Take Evening Primrose Oil or Black Cohosh capsules daily, you can get these from supermarkets and health stores to help regulate your cycles. If you get cramps use Clary Sage oil from a health store, massaged into your tummy it's better than painkillers, also it supports estrogen production and acts on the pituitary gland to help regulate your cycles. Some missinformed people suggest the pill, this doesn't regulate your menstrual cycle at all, it suppress it, so although periods come at regular times it's only as they're forced. This stops you learning from your menstrual cycles, which is important in caring for sexual health, it means problems get covered up instead of treated, and the pill has side-effects. The negatives outweigh the positives, if offered this treatment make sure you make an informed decission.
Take a look at Natracare's Monthly Matrix, an interactive page that shows you what happens during your menstrual cycle http://www.natracare.com also look around Scarleteen which is a great sex education site http://www.scarleteen.com
As for cramps, rather than treat cramps you're better off preventing them, look at books and sites to find what works for you.
Don't use painkillers if you can help it, appart from the fact that they don't cure cramps but at best just numb the pain, they can actually make cramps progressivly worse over time. Rather than painkillers get some Clary Sage oil - it's an essential oil you rub into your stomach, it works better than painkillers and you don't build up immunity, it also supports estrogen production and acts on pituitary gland to prevent cramps in the first place - it's pretty much the best way to ease menstrual cramps.
Look into different herbal treatments to prevent and cure cramps, fresh is best but tea bags are fine, just don't boil as this effects herb potency and use just before and during your period. Look for women's blends or yarrow, lemon balm, black haw, chamomile, red raspberry, wild strawberry, valerian, and raspberry leaf works very well to stop cramps. There is Bupleurum (xiao yao wan) which is a Chinese medicine, you take 3-4 tablets 4 times per day for two weeks before your period is due, buy from Chinese herbalists. Take Evening Primrose oil or Black Cohosh capsules daily to regulate hormones to prevent cramps, you can get these from health stores and supermarkets too. There are lots of other helpful herbs, try talking to health store staff for help, and look at Sister Zeus http://www.sisterzeus.com
Heat is good to increase blood flow to pelvic organs, some people prefer cold, if heat works try a relaxing hot bath with lavender oil, also masturbation will help as during orgasm a the hormone oxytocin is released causing uterine contractions, orgasm also means relaxation and improved circulation. Avoid tampons like the plauge, they're very bad for vaginal health in turn effecting menstrual health, they also expand adding to bloating and hitting the cervix causing more pain, instead use options like pads, softcups or menstrual cups which are safer and generally much better than tampons anyway, and you should notice a difference in cramps and flow.
Excercise to stretch abdomen and improve breathing, this video of yoga postures for cramps - http://www.videojug.com/film/yoga-postures-for-menstrual-pain-relief or try stretches like kneeling, sloping your back, lean on your arms and rest your head on the floor, rest and relax in this position, or lie on your back, hugging one knee, roll your body to the opposite side and back. Complimentary treatments can help such as Acupressure, you can use basics yourself, simulate points BL24 (Qihaishu), BL25 (Dachangshu), BL30 (Baihuanshu), BL31 (Shangliao), BL32 (Ciliao), BL34 (Xialiao), CV2 (Qugu), CV6 (Qihai), KI5 (Shuiquan), and KI6 (Zhao Hai). See this video showing you how to use certain points to stop cramps http://www.youtube.com/watch?v=afYHM9y4lJY - this will ease cramps pretty much straight away!
To prevent cramps diet is a huge factor, small changes before your period can make a big difference, but it's so complex you'll need to research it yourself to know what foods to eat and why. For now get a balanced diet of nutritious foods rich in essential fatty acids and low in refined sugars, refined carbohydrates and hydrogenated fats, avoid foods that make you bloated such as salty foods, when bloated drink water with a little lemon juice as a natural diuretic. Some find dairy, caffeine and especially soy cause problems – although coffee is good for reducing menstrual flow. Try taking a good multi-vitamin, omega 3 or flaxseed oil (approx. 500Mg 2-4 times a day), B vitamin complex with 100mg of vitamin B6, Magnesium (100mg every 2 hours during menstruation), and vitamin E in d-alpha tocopherol form (50mg 3 times a day). It sounds complex but for a start try a multi-vitamin daily and increase magnesium, calcium and potassium during your periods which are needed by your body to help uterus shedding. Cell salts are a good natural mineral to take as your body needs these for basic functioning so if lacking it can cause problems – when you get cramps take them every 5-10 minutes until the pain stops, see http://en.wikipedia.org/wiki/Cell_salts
You can treat cramps yourself but if that fails see a doctor, for advice or testing to check for more serious problems such as endometriosis. You can ask your doctor for an anti-inflammatory drug such as Mefenamic acid to block the the synthesis of prostaglandin F2 alpha, a hormone responsible for cramps, you take these 3 times a day for the first 4 days of your period.