There’s a couple of simple things you can check to start with (I’m assuming Windows 10 here). Click the images for a bigger version.
The processes tab of task manager will show you what’s running now, how much CPU and memory they’re using. There will be a lot of background processes you don’t recognise, chances are they’re a normal part of your computer’s operation, but make a note of anything that seems to be using a large amount of CPU or memory so you can figure out what they are.
If there’s nothing obvious here, you also have the details tab. This is less descriptive, typically just showing you the name of the executable running, but you can see the cumulative CPU time it’s taking up. Right click on one of the headings at the top (e.g. name, status) and choose “select columns”. Tick these options:
You’ll then be able to sort the list by which processes are taking up the most resources and again look for anything out of place. e.g. here I can see “MsMpEng.exe” (Windows built in anti-virus) was earlier using a significant amount of resources but currently isn’t, probably running a scheduled scan or doing updates when I turned my PC on.
If this doesn’t turn up anything I’d recommend running virus scans, but I’ll leave it to someone else to suggest the best way to do that.