Im 12 and I need a job because I want to start purchasing my own things where could I work

Where you live at, makes a large difference, In the US, there are laws that restrict many things you can do. Often 12 year olds will baby sit, animal sit, dog walk, deliver news papers ( not as popular as it use to be), mow yards, rake leaves, shovel snow.

I know one 12 or 14 year old that designs web sites

For being 12 hun the only thing you could do is working for friends or family like mowing lawns, baby sitting, or washing peoples cars. Now that winter has set you could even go around your neighborhood and see if anyone needs leaves raked.