https://psref.lenovo.com/

Put your model number into the top search field to bring up the spec sheet for the model in question.

It will tell you the ram that's already installed to mirror it. Chances are you'll be looking at either 8gb or 16gb, sodimm ddr4 @ 3200mhz. Brand doesn't really matter too much but it must be sodimm not dimm, and make sure you don't get lower MHz then what's already installed as it will make all stocks default to the lowest MHz speed

For laptops, the most important thing is to check how many ram slots your laptop has. If you're sure that there's enough slots, then continue on.

For RAM, there's a handful of important aspects to keep in mind:

Size: either SO-DIMM or DIMM. Laptops almost exclusively use SO-DIMM, so you'll probably want to search for SO-DIMM

Generation: you're probably using DDR4 (4th generation). Generations aren't interchangeable, so stick with DDR4. (DDR5 won't fit in your laptop)

Memory size: depends on what you want, tbh. But I expect 16 GB or 32 GB to be fine. Try to split the memory across multiple RAM sticks if possible (ie, 2 sticks of 8 GB is usually better than 1 stick of 16 GB)

Memory speed: not a particular concern, but the standard is 2133 MHz. Any higher than that and you'll need to go into your BIOS to turn off the memory speed limit (the BIOS automatically caps your memory speed at 2133 MHz unless you turn off the limiter). If you don't know how to do that, then honestly anything 2133 MHz or higher would be functionally equivalent

Memory latency: really not a concern, but it's usually listed as CL14 or CL16 or whatever. It's the time that if takes for the RAM to give information after being prompted. Smaller number is better, but again, it really doesn't matter

Price: DDR4 is getting pretty cheap these days, so I wouldn't expect this to be a big concern. But higher speed, higher size, lower latency RAM tends to be more expensive. Just keep that in mind.

Alright, I'm a software engineer and PC building hobbyist, so by no means an expert when it comes to this stuff. But I've added and replaced so much RAM over the years, that I think I can help you.

Overall, what you need to know is the DDR version, if they are DIMM or SODIMM, the frequency and the capacity.

I usually figure out the DDR type by simply looking at whatever was in the machine previously, or checking out the motherboard specs.

I've never encountered other than SODIMM on a laptop. The same cannot be said for a desktop, which can use either depending on the motherboard.

If you make sure the DIMM type and DDR version matches your motherboard, the stick will fit in your machine.

For the frequency, I generally try to make sure all sticks run at the same frequency. This can also be tweaked in the BIOS.

When it comes to capacity, it depends on your needs and budget.

Good luck with your upgrade!

Websites like crucial.com let you enter in the details about your computer and will tell you what kind of RAM to get. Crucial has good RAM, but you can still use that information to buy similarly spec'd RAM elsewhere.