First off, how large is the Used
capacity on your current C: drive?
Next, what is your budget?
These two questions will guide your SSD purchase. If your current used space on the C: drive is under 500 GB or even under 256GB then you can get an SSD of that size. If your current C: drive is much larger and money is no object, then you can get an SSD up to 4TB. Or something more modestly priced at 1TB. You can also get a smaller SSD drive if you move your User Files to a different drive before you clone the C: drive
Once you get your new SSD. place it in the computer and your current C: drive, many SSD drives come with a free version of Acronis cloning software or download Macrium Reflect
or any other Cloning program. After installing the software choose to make bootable USB Recovery Flash Drive from the cloning program.
Boot off of the Flash Drive and choose to clone your current C: drive on the mechanical drive (the source drive) to the SSD (the Target or Destination drive) After it's complete, make sure the SSD is the First Boot Device in the Bios and it should boot from it. Once everything is working great and you feel comfortable, you can Format the mechanical HDD and use it for Data Backups. You can also, boot off of the clone program USB Drive and choose to make an Image file of your current setup and save it to the mechanical drive as a clone backup in case of a disaster.