To answer the question in your subject and not the text, there are two common thread sizes in use - 1/4" x 20 and 3/8" x 20. Cameras usually have 1/4" tripod mounts.
Don't forget to budget for the head and a QR system. My tripod head costs about double what my tripod costs. And a QR system - invaluable. Makes it so much more convenient to use the tripod, which means you're more likely to use it.