Pick a Size
Chances are, you already have a pretty good idea what size planner you are interested in using. Some of your choices are notebook size (8½ x 11), half-size (8½ x 5½), personal size (3.7 x 6.7), compact (4 x 6) and pocket ( 3.2 x 4.7). I would suggest starting out with an inexpensive version of the size you think you will go with.
For example, if you are thinking about a half-size planner, purchase a 3 ring binder in that size, or an inexpensive undated planner. No, they are not as pretty as the more expensive planners but it is good to see if you are going to stick with this size before handing over your hard earned money. I would hate to be stuck with a planner that I didn't really like just because I paid a lot of money for it.
Get Some Inserts
Just like there are lots of planners to choose from, there are also many inserts to choose from. Don't go crazy in the beginning; choose some week at a glance and monthly insets first. Start writing on them Find your groove. I suggest Giftie Etcetera's blog for developing a planning system that is efficient and practical. You will find lots of free inserts right here on my own blog.
You'll Need Some Dividers
I have made my own as well as bought them. If you are a true beginner and not very crafty, Amazon has basic dividers to fit most planners. Start with those and write in your categories. I suggest: Calendar, Projects, Future, Tasks and Files (or Notes). Giftie suggests waiting a month before adding more dividers, which I think is a very good idea!
Write Things Down
This is how I started! I opened up my monthly pages and write down items that had a due date: bills, doctor's appointments, church meetings, due dates, etc. I would expand my week by using the week on 2 pages inserts; I still write down due dates, but I also add tasks that need attending to ("call the pharmacy on Tuesday", etc). My week on 2 pages have more or less replaced a dedicated to-do list
Review Your Usage
Do you like the size you chose? If so, and you started out with a basic binder, now might be the time to consider a more permanent home. I went with a Daytimer Binder - not cheap but not top of the line either. Are the pages working for you? How about your planning method, are you getting things done. After you have put some mileage on your planner, you should have a better idea of what is working for you and whether or not you need to make changes.
The best advice is to start small and just jump in. Your style and methods will mostly likely change and evolve a number of times in the beginning.