If you want to talk about the purpose of something, use "to/in order to/so that". For example, I use my computer to surf on the net/ I use my computer in order to surf on the net/ I use my computer so that I can surf on the net.
However, "for" is also possible to talk about the general use of something. For instance, we use a knife for cutting. We use a key for opening the door, but I use a knife to open the door (an unusual use of something).