I would recommend the 250GB Samsung 850 Evo.
Still the best value SSD on the market in terms of price to performance.
If you want to install an SSD, simply download a migration tool (preferably from windows) and select the drive you would like to transfer your OS to.
Make sure that you only transfer the operating system, and leave behind other files on the HDD, which you should then put as your secondary drive in the new PC.
Put the boot drive (SSD) on sata port 0, followed by 1 for the HDD.
It only makes games load faster, not run faster in case you were wondering.