|MadSci Network: Physics|
Your question "how does water turn into ice?" is very advanced. Since I do not have more information on what you already know about I will give you a simple answer. However I also encourage you to visit wikipedia or better yet your library to learn more about this very interesting subject.
The simple answer is this: Water molecules find it very easy to stick to one another in different ways. When they are liquid they have a temperature which means they have captured heat and this prevents them from sticking together in solid sheets. Instead they jiggle around. It would be similar if you and your friends were in a room running around at full speed wearing shoes with velcro straps. Because you and your friends are always running past one another very fast in this room, it is very unlikely that the velcro straps from your shoe will stick to the velcro straps on your friend's shoe. Of course you can link arms with your friends for a moment or two and then break loose again to keep running. But you will never make a solid stable structure this way. You and your friends represent water molecules in liquid.
Now put your liquid water in the freezer. The freezer absorbs the heat from the water so the water molecules are able to slow down. Eventually some of them stick together and start to form a pattern called a crystal. Soon other water molecules fall into this pattern and the crystal grows. It would be like you and your friends walking around the room very slowly; eventually some of you will bump together and your velcro shoe straps might stick together.
Linking arms and velcro straps are my analogies for molecules sticking together, what are technically called chemical bonds. The freezing of water is a very interesting example of the consequence of understanding that the matter around us is made from atoms and clumps of atoms called molecules.
Try the links in the MadSci Library for more information on Physics.