The Arch Linux Wiki is full of excellent information. It’s not for beginners, though.
Run top in your terminal to see what’s taking CPU.
Run top -o RES (or what’s easier, run top and then press M while it’s running) to see what is taking up RAM.
… though unfortunately, it’s mighty probable that the only significant consumer of memory and CPU is your browser. Get uBlock Origin, it helps web pages be lighter and eat less resources. Don’t open too many tabs at once - learn to use bookmarks efficiently, instead (folders, bookmarks toolbar and whatnot).