If you are thinking about remodeling your bathroom, there are many things to consider. Most importantly, if you are planning to make structural changes to your room, you will want to have a licensed contractor at the helm. He or she will obtain the necessary permit, and organize the other trades involved.
If your are just replacing tile, or other non-structural changes, you may simply be able to hire a handyman or tile pro. Be sure to check with your local building official regarding what does need a permit.
Whatever the case, if you are having any plumbing or electrical work done, I always recommend having a licensed individual take care of those needs.
Next, you will want to discuss, with your pro of choice, what designs, tile sizes and materials will work for your particular bathroom. It is very helpful to meet your pro at the tile store. There you can get on the same page with your dream. Have some pictures of other work that you like the look of. Sometimes the image you have in your mind can be very different from the one in the pro's mind.
Write down all of the particulars for your bathroom. What backer board will be used; what form of waterproofing; tile size and model number (insist that all matching tile is from the same dye lot); grout joint size; pattern; fixtures; position of things such as a towel bar or seat; acceptable level of quality expectations (ie. no more than a 1/16"-1/8" variance).
Ensure everything is in writing - before you agree to anything!