The XMRig developer provides pre-built binaries for Ubuntu Bionic (18.04) and Focal (20.04). They may work on other Ubuntu versions, and on other distributions, but this is not guaranteed.
Scroll down to 'Assets' until you see the correct version for your system. Download this file, extract the archive somewhere memorable, like your desktop.
Binaries are available for multiple distributions and operative systems, but you can also build the miner by yourself. Take a loot at [XMrig's docs](https://xmrig.com/docs/miner)
Most pools are <abbr title="Pay Per Last N Shares">PPLNS</abbr> pools, which means that you only get paid when a miner on the pool finds a block. If the pool you are mining on is small, this can take a few days to weeks.