Ok, I'll take a deep breath and plunge in here:
*clean the room and the desks. If your school does a summer "clean," that may just mean dusting and wiping things down. If not, it could mean more. I spent several days cleaning desks for the first classroom I moved into. Besides throwing away trash and rotten food inside, I was scraping gum of every imaginable surface and scrubbing grafiti.
*Organize the furnishings the way you want them. You might like them the way they are, you might move student desks around, or you might move every thing in the classroom to get a set up that is workable for you. Make sure you can see the whole room from anyplace you might happen to be. If you plan to spend time at your desk when students are in the room, don't put it back in a corner somewhere. Put it where your presence will be prominent.
*check your technology resources; do you have any computers in your room? What software is available? A printer? While you are at it, check out your library, computer lab, and other resources at your school.
*Make sure you have all the supplies you will need for the first week. That may mean getting them from the school supply cupboard, doing a district warehouse order, buying them yourself, or a combination of all of the above, depending on your district's policies and supply budgets.
*Now "decorate." Cover your bulletin boards with butcher paper and borders. Put up signs, posters, charts, etc. that you will be using.
Bring in whatever personal items you will use, and set up all those supplies. Get your textbooks, unless your school checks them out directly to students.
Write a welcoming letter to parents, introducing yourself and letting them know what your classroom expectations are, upcoming events, and what their children should bring to school on the first day. Mail the letters to arrive a week before school starts, if possible.
Get a copy of the daily schedule, with lunch, breaks, etc. on them. Also get a list of planned field trips, special events, or school wide programs that will be happening throughout the year, and get your roster. Make sure you get information about your class like: allergies and health concerns for individuals, IEPs, Student study teams, special ed services, speech, gifted ed, ELL, etc. If possible, get a look at their permanent records to see who is brand new to your school, who has a history at your school, and what special services anyone might need.
Meet with your grade level, your teaching team, or who ever you will be working with to talk about things like your daily classroom schedule of when each subject or section will be taught. That may be an administrative decision, a group decision, or a personal decision, but it's good to talk to others about managing the schedule anyway. Deciding how much time to spend each day on each subject, when during the day to do so, and how to squeeze in "extras" can be a challenging job.
Now that you have your roster, your supplies, and your schedules, set up your grade book and your plan book. Your grade book may be a hard copy, a computerized version, or both. If you want to, now would be a time to make a seating chart.
Finally, it is time to plan your first week. Or, your administrator may require a "yearly plan" to be posted or submitted; if so, do that first. Take your texts and your content standards and schedule in blocks for when you will teach what; then you can make sure your plan book stays on schedule.
For your first week, give some extra time for things like getting organized and getting to know each other. Presentation of rules/expectations, grading/homework policies, a school tour, passing out supplies, some games or exercises to get to know people, all belong in the beginning. This week is also where you may do some assessments to get to know where your kids are at and what their needs are; not necessarily for grades, but for your own information. I keep a portfolio of such assessments for each student. Do introductory and warm-up activities for each subject in addition to the assessments the first week, and you should be up to full speed by the second week.
Make sure you've accomodated time before and after class for parents that first week; they will be there when the opening bell rings, and again after school, and maybe during the day if your school encourages them, to see where their kids are going and get to know their teacher. Many will want a few minutes of your time to tell you all about their child's individual needs, gifts, quirks, etc.