First download the SERVICE MANUAL, which gives step by step instructions. You will need a small phillips head screwdriver (very small, you can see the size by looking at the screws on the back of the laptop).
Then depending on your budget buy a 2.5" SATA SSD. I usually go with Samsung, but there are cheaper brands available. I start by looking on Amazon and then check Newegg and a few other places for the best price considering shipping (if any).
A HDD on a Dell is pretty easy to replace. Most Dell models have the HDD in a small cage with a few screws (as you take them out put them in a container -- they are M2 and M2.5 short screws that are easy to lose). You simply take out the cage, unplug the connector, remove the HDD from the cage, then reverse the order as shown in the manual, which has pictures and very good details.
And before starting back up any data that you can or want. You will need to do a clean re-install of the OS on the SSD, so have everything ready including the drivers from Dell that you can get from the DELL SITE. Click on the manual download button and select the OS and then the drivers that you need. I do not recommend using the automatic find it for me function.