Hi,
Here’s the [https://downloads.dell.com/manuals/all-products/esuprt_laptop/esuprt_vostro_notebook/vostro-1550_service%20manual_en-us.pdf|owner manual] for the laptop.
Go to p.29 to view the necessary pre-requisite steps and then the procedure to remove the HDD.
Here’s a link to just one [https://www.crucial.com/compatible-upgrade-for/dell/vostro-1550|supplier] of compatible RAM modules and SSD for the laptop. It is only to show you what is compatible and how much it may cost. Note that a 9.5mm adapter is required with the SSD so that it will connect properly in the laptop’s drive bay.
There are other suppliers online as well that may suit you better.
Also usually the SSD comes with instructions on how to transfer the HDD information to the SSD.
Hopefully this is of some help.