What is your budget and currency? $500-$600
Do you need a monitor? Already have one :asus vg248qe
Do you need a keyboard? No
Do you need a mouse? No
Do you need an operating system? No
What will you use the PC for? I plan to use it as my main everyday computer and also play some pc games as well. I would like it to be powerful enough to run games such as CS Go, H1Z1 and Dayz at good frame rates. And will also use it to code as well if that effects its performance and I also want it to have 16GB of ram.

[ Register or Signin to view external links. ] / [ Register or Signin to view external links. ]

CPU: [ Register or Signin to view external links. ] ($169.99 @ SuperBiiz)
Motherboard: [ Register or Signin to view external links. ] ($46.98 @ Newegg)
Memory: [ Register or Signin to view external links. ] ($55.89 @ OutletPC)
Storage: [ Register or Signin to view external links. ] ($50.89 @ OutletPC)
Storage: [ Register or Signin to view external links. ] ($39.99 @ Amazon)
Video Card: [ Register or Signin to view external links. ] ($149.99 @ Newegg)
Case: [ Register or Signin to view external links. ] ($29.99 @ SuperBiiz)
Power Supply: [ Register or Signin to view external links. ] ($49.99 @ B&H)
Total: $593.71

Ask Newegg to update the BIOS on the motherboard for you so it's compatible with the CPU. 16GB RAM is going to take up like 1/6th of your budget, it just doesn't make sense. Get 1x8GB for now and you can add another 1x8GB stick in the future.

Do i need to choose a cpu cooler or it comes with one already?

The i5-7400 will come with an included stock cooler. It's not great, it's pretty loud and ugly but it'll do the job. You can replace the cooler later if you want, but you might as well use the stock cooler for now to save $20.