Can I upgrade my MacBook Pro Retina? So I saw around on the internet that people have changed their HDD to an SSD  on their MacBook Pro which people say helps in speed.

This might seem silly to ask here for some of you but I like to get direct answers from someone with better Mac knowledge than me.

But if I were to do so, is there a certain type SSD I need and what would be the price? Depending on space. And is it easy?

I have an Retina MacBook Pro 13" 8GB RAM, 2,6GHZ, 128HDD (Mid - 2014)
Unfortunately I cannot comment on the other answer.  Ian is correct in that the drive inside the new rMBP's are solid-state drives, however, you can upgrade the drive to a larger drive yourself, but in doing so you may void the warranty so make sure that you verify that first, otherwise, you can take it into an Apple Store and they likely will upgrade it for you.  Either way, this will only give you more space and chances are will not increase your read/write speeds.

Here's the link to the instructions on replacing the SSD.

https://www.ifixit.com/Guide/MacBook+Pro+13-Inch+Retina+Display+Mid+2014+SSD+Replacement/27849

Your only obstacle might be finding one.  Currently, the SSD's that Apple uses are made by Samsung and use a PCIe connection.  They are very difficult to find as they are not yet on the mainstream market.