First of all check what interface you have on your system to connect the SSD to.
Is it very old PATA interface: 
still old SATA interface: 
or an m.2 interface and is it M-key or B-key: 
B-Keys are quite old and support the SATA SSD. M-keys are the new NVME SSD.
Now you need to determine what size of m.2 SSDs can be installed on your device and you need to check how much space your laptop has given for it and if there is a screw holder to hold them down. Also check if you have a screws since many laptops/motherboards don't come with that extra screw nor the new SSD.
m.2 2280 is the most common size and the cheapest to get SSDs for.
When looking for an SSD visually check the notch on them which will tell you if they are SATA or NVMe.
on m.2 NVME you have to decide if you want Gen 3 or Gen 4 SSD. Mostly you will find that both don't have much difference in the same class of SSD and it makes more sense to get Gen 4.
Now you know which interface you have: PATA, SATA, m.2 SATA or m.2 NVME. You can start looking at the SSDs below.
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