High rent prices in England have forced lots of people to share houses. What this means is that you rent your own bedroom in a household and you share the common rooms, such as bathroom, kitchen and lounge (if this exists!) with the people who rent the rest of the bedrooms. This is usually a cheaper option than renting a flat or a house.
The best website to start your search from is Spare Room and it is free of charge. There are several other sites too; just google them. You will probably need to subscribe for a small fee. If you decide to use Gumtree make sure you avoid scams (read the useful tips section). 

If you like your privacy or you are moving to the UK with family or friends, then renting a flat or a house is the best solution. Renting your own home is generally more expensive than renting a room if you are on your own. If you share the rent with your family, though, then it can be cheaper than renting your own room. It also depends on the location. A room in London costs as much as a house in a city in the north of England! If you intend to rent a house, it's easier to do it through an agency. Instead of going to individual agencies, you may be better off using websites such Find a Property, Zoopla and Right Move. Right Move has a very helpful gadget, draw-a-search, where you define the area you are interested in on the map and they show you all the places to rent in the area. When you find the flat you want, you then contact the agency. The agency will not do the work for you for free. Make sure you ask them about their fees before agreeing on a flat.