Weekly food plans/Budget with low repetition?

In an effort to reduce my waste and expenses (as well as get some control over my diet...), I've been looking at weekly food planning and trying to budget for stuff...

I'm lost. My typical shopping is either getting an idea for something to cook then buying the stuff, or getting the ideas while shopping for other things. This has often resulted in having to throw stuff out because it doesn't actually get used.

I'm hoping to hear from people that are successful with actually sticking to a weekly plan and budget... How do you do it?