First to get to and from Langkawi, you can get there by Malaysia Airlines, SilkAir(managed by Singapore Airlines) and Air Asia.  You can book them online at airasia.com and the prices are incredibly low compared to the competition.  Keep in mind that Air Asia has a luggage rip-off policy where they limit you to one checked bag per person and 20 kg per bag.  They will charge extra for "overweight" luggage although if you're a couple they will combine the weights and limit you to 40 kilos total per couple.  Carry-on luggage is not normally weighed unless it appears to be way too heavy or oversize. They do allow 7KG and have been checking lately.

OR by ferry from Penang, Kuala Perlis, Kuala Kedah or Satun, Thailand, check out the site for prices and schedule langkawi-ferry.com/

Rental cars are very cheap but you get what you pay for, BE VERY CAREFUL (SEE COMMENTS ON TRIP ADVISOR)  Car hire on Pantai Cenang generally start at 50 ringgitt a day for a small car and increase to about 80/90 ringgitt a day for the larger models. Malaysia drives on  the left so be careful when you first start out - especially at the first turn onto the major road by the airport!  Petrol is subsidized and cheap in Malaysia.

There is a nice resort very close to the airport on the beach but it's behind a breakwater so the water is stagnant and if you want to swim or snorkel don't bother.

A very major problem on this island and others is jellyfish.  An alarming number of tourists end up in the hospital with a jellyfish stings.

For over $120 per person you can get a boat ride out to a reef which is relatively free of jellyfish but that's very pricey for a short boat ride.

There are few ATMs on the island and they usually work fine.There is 2 ATM at in Pantai Cenang at Underwaterworld - in high season expect long queues. There are a few moneychangers along the main street (only street) in Pantai Cenang.  If that Atm is out of order (empty) then there are two more ATMs at the airport and another at Telaga Harbour. Apart from a small Maybank at Telaga hrbour all the banks are in Kuah town. 

Dining in the Pantai Cenang can be a hit or miss affair. With so many tourists just passing through for a few days some restaurants feel they don't have to make the effort to attract repeat business and more emphasis is placed on decor or location than on the quality of the food. Compared to mainland Malaysia restaurant prices are drastically overpriced with many restaurants charging western prices for western food. If you want an authentic experience (and are not vegetarian) stick to the local stalls which offer the usual standard dishes at reasonable prices. The Laksa vans that park along the road between the airport runway and the ocean are worth checking out. 

On the "airport side" of the island they have hourly excursions on small boats they call "island hopping" where they take you to a fresh water lake on one of the island where you can safely swim and then on to feed the Sea Eagles and then spend time on a small island beach.  The price is around 15 USD dollars per person and they will usually pick you up at your hotel although you save a bit if you drive yourself to the dock.

They have some pricey beach resorts which look nice and some very cheap ones.  Even the cheaper option is clean and comfortable if all you need is somewhere to sleep. There are plenty of restaurants and bars on Pantai Cenang beach both local and European, along with a swinging nightlife.

If you need more information on Langkawi, try the Langkawi forum where there are plenty of residents who will answer your questions with accurate knowledge.