Can I deduct expenses for items purchased before business started but are used partially for business? If I need to depreciate these items, how do I do it?

I began doing independent contracting this year, and use items I purchased (for personal use) before I began the business such as my laptop and cell phone.

Can I deduct these expenses? I have seen some references to needing to depreciate the item and only claim the percent of it used for business, but I don't know how to go about doing this and I can't seem to find any specific instructions on how to depreciate or where to put the business use percent of the depreciated value. I imagine there's an IRS form somewhere that would walk me through the calculations/regulations but I have been unable to find it. 

Thank you!