Hi Ribena
Friends of ours fostered a little boy two years ago when he was 5. They had no children of their own at the time. The lad already been to 6 other foster homes. They had a very difficult experience to start with, in that his natural parents are still around and make life difficult for the child by 'playing mind games with him'. They told him when he would visit that his foster parents didn't really love him, that they only had him to get paid and that when they had their own child he would be thrown out again. Thankfully things have settled down in this case and everything seems to be going well.
Like the other posters have mentioned you need to be a certain type of person to do this ... and if you are I hope all goes well. Make sure you go into the job with your eyes wide open ... and try to prepare yourself for the consequences if it doesn't work out.
Good luck.