You will need at least a CPU, Motherboard and RAM. Most likely you will also need another Windows license. Without detailed specs or knowing what your needs for the computer are. No one could give you more than a general list of parts.
By detailed specs I mean. All components installed (PSU, hard drives, SSD, optical, video card, other expansion cards, &c). What case you have would be helpful.
Something like this would cover the average persons needs. If you already have a dedicated GPU. I would go with AMD Ryzen instead.
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel - Core i5-7500 3.4GHz Quad-Core Processor ($187.59 @ SuperBiiz)
Motherboard: Asus - PRIME B250M-A Micro ATX LGA1151 Motherboard ($65.99 @ Amazon)
Memory: Patriot - Signature Line 8GB (2 x 4GB) DDR4-2400 Memory ($62.99 @ Newegg)
Total: $316.57
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-08-01 17:07 EDT-0400