How do I get more storage space?

I have several questions that I ask in the course of this description. I will now describe the problem.

For weeks, my phone (Huawei p9 Lite, if someone needs it for the answer) with 97-99% space occupied. I do not know why it could be. I only have 5 apps installed that are very important to me. I have already deleted superfluous ones. Every 20-30 minutes comes an error message, even if I'm doing nothing. I can't click it off and I have to clean up the memory, which I often do manually. Nevertheless, it remains at 99% and I have to restart the device. It's pretty hard for me to always delete all videos, pictures or apps because it's related to memories. I have absolutely no idea of any technique, so I ask this question. If I order a memory card with 32GB on the Internet, then I just have to open the flap on the side (with other devices like Samsung the "cover") and insert this card? I can't look now, but is not there already the SIM card in there? Where should the memory card go? If it is useful as an info, I have a total of 16GB and 434MB of it. Free. If I order this memory card and use it correctly, do I have 32GB or 48GB available?

It would be great if I get an answer that explains it completely.

Delete photos or apps or outsource apps that you net more often use that will dan "deleted" but the icon remains and if you need them again, all data will be restored

The part has a SD card slot for up to 128 GB.https://www.inside-handy.de/handys/huawei-p9-lite

So just buy a microSD, microSDXC or microSDHC and insert.