What Thor comic book should I start with?

Well if you want t start with Thor, and don't have a vast knowledge get the Thor omnibus, it's 100 dollars but it's a thick hardcover collecting a lot of his early appearances in Tales to Astonish.Or see if you can search for any of the Marvel Masterworks Thor series, which are reprints. Now if you can't do that, you can jump on from the newest Thor series, and play catch up. As the newest series began back in 07 with Thor vol 3 issue 1 you can catch up relatively quickly, and just use sites like comicvine, wikipedia, and marvel database to answer any questions you have.And the series recently picked up the original thor's numbering.

Or you can wait until the mighty thor is collected in trade format, which will let you read thor without having a deep knowledge. But I just recommend diving in, and picking up whatever looks good to you, with the internet you can fill in any gaps in your knowledge.

Pick up the first issue of thor it should be "a reprint"