So by “fix” I assume you mean you want to replace the old laptop HDD with a new SSD?
If your laptop has a HDD, it most likely has a 2.5” data HDD. You can find 2.5” SSDs pretty much anywhere that sells electronics. Best Buy, Amazon, and I think even walmart sells them. Not to mention the countless online retailers.
Upgrading a laptop to an SSD is typically one of the easier things to do on a laptop but it simply depends.
What is the brand and model of your laptop? This will help up determine what is compatible with your laptop, as well as finding a guide on how to access the HDD and replacing it a new SSD.
If you also want to retain all of your data, consider using a desktop computer to clone your old drive to the new one or use a usb to sata adapter to plug your new drive into your laptop and clone it that way. This adapters are about $10-$15 on Amazon.