Firstly you (and your customer) needs to consider the specification and possible issues - page size, number of printed colours, type of paper, number of pages, whether it's to be ilustrated and, if so, the source and quality of the images, timing, distribution and, of course, not forgetting the budget!
Regarding number of pages, asuming it's to be a printed magazine, you need to decide whether it's to have a "self cover" (that means having the mag printed on the same paper on all pages) or have a separate cover (the inside printed on lighter paper and the cover printed on heavier paper and maybe be laminated or varnished). You can't have a 35 page printed magazine, so consider in terms of units of 4 or 8 pages. For example, 32 pages or 48 pages for a self-cover issue or alternatively say 32 pages plus a 4 page cover for the latter specification.
Having agreed the above you need to consider the design - (How are the text and pics to be supplied?) you can produce a grid, consider typefaces, the masthead and so on. Probably a good idea would be to produce some sample pages using placeholder text, if the final text isn't available, so you can use this to show and agree your approach with to your customer and maybe with your printer too before proceeding.
You should also get a book on how to set up InDesign documents and a lynda.com course would be invaluable. for example: http://www.lynda.com/Bridge-CS4-tutorials/designing-a-magazine-layout-hands-on-workshop/46 818-2.html