Getting fitted at a store for shoes advice?

marisposa

So my feet and now my knees are starting to hurt ive gone through 3 pairs of shoes I have heard people say on here go to a running shoe store and get them to fit your feet. I feel a bit intimidated being a bigger girl going in since im clearly not a runner. What is it they do to measure your feet and how much do the shoes generally cost ? any advice would be great.