Before I list out what I'd recommend let me say this, treat that $900 like you are spending it on a hobby.  Don't expect to get rich from it and enjoy the ride.  That will make your experience a lot more fun. 

For your first rig I'd suggest the build below.  This is strictly based on what is on sale at Microcenter right now so you can spend most of your money on GPUs.

CPU -- http://www.microcenter.com/product/486115/FX_8320E_Black_Edition_Vishera_32_GHz_8_Core_AM3_Boxed_Processor -- $80 with a $40 rebate and save $30 when bundled with a motherboard.
Mobo -- http://www.microcenter.com/product/484530/GA-970A-DS3P_FX_AM3_ATX_AMD_Motherboard -- $70-$30 for bundling with CPU.  Could support 5 GPUs later
RAM -- http://www.microcenter.com/product/460070/4GB_DDR3-1866_PC3-14900_CL13_Single_Channel_Desktop_Memory_Module -- $40
PSU -- http://www.microcenter.com/product/467824/750_BQ_750_Watt_80_Plus_Bronze_Semi-Modular_ATX_Power_Supply  -- $80, $65 after rebate.  This will be one of the first parts to replace once you are making your money back.  This is just a decent brand power supply to get started with.  Should at least support 3 GPUs.
Storage -- http://www.microcenter.com/product/485586/32GB_SuperSpeed_USB_30_Flash_Drive -- $11 flash drive to put your OS on.  I'm a big fan of simple mining OS, it's $2/month and offers a free trial of 7 or 14 days I think.

This should leave you with around $650 for GPUs.  I'd recomend throwing in a few more bucks and buying 3 of these --  http://www.microcenter.com/product/478365/Radeon_RX_570_ARMOR_Overclocked_4GB_GDDR5_Video_Card  --