When Hiring a Contractor...
Educate Yourself - Shop for tile so you know what you want and what it costs. Contractors can often get better prices, so you may want to use your contractor's supplier anyway, but having an idea of what your going to pay for materials is still a good idea.
Know What You Need Done - Most tile jobs consist of more than just tiling. You need to make sure you take into account the cost of not only the tiling, but also the cost to rip out any pre-existing laminate, carpet, old tile, etc. Make sure you know what all your costs are.
Don't Assume - Make sure you discuss everything with your contractor. If you aren't sure if the contractor is paying for tile, ask him. Don't assume your contractor knows exactly what you want. Be as detailed as possible in order to make sure you get the job you want.