Pizza man, I will assume that you bought the property in 2008 and was using it as your primary residence. Now you wish to use it as a rental.
The basis is the
lessor of fair market value and your cost basis (your purchase cost + improvements).
You can determine fair market value however you want but you are going to have to justify your technique should you be audited. If possible, comparable sales is an easy way. You can do it yourself or a realtor might be kind enough to do it for you.
See page 10 on
http://www.irs.gov/pub/irs-pdf/p551.pdf
Also, keep in mind, you don't depreciate the whole amount, only the improved (structures, etc) part. You do not depreciate land. It is up to you to determine what percent of your property is depreciable. Again, use any technique you want, you just have to justify why you used it.
Real Estate taxes is not difficult but it is complicated. There are generally reasons the laws are they way they are. The reason for using the lessor of FMV and your cost basis is if your property lost value after you bought it, you can't convert it to a rental, use your (now overvalued) basis, and then when you sell deduct the loss. You can only deduct losses that happen after you convert it to a rental.