The quickest and easiest way to find the right cartridges is to enter your printer model number in the search bar at the top of each page on the website:
In most cases you will just need the alphanumeric part, for example if you have an Epson Expression Home XP-205, then "XP-205" (or even "XP205") will be enough.
If you have a HP printer, it is advisable to include the series (for example, "DeskJet" or "PhotoSmart") as well, because there are some HP numbers that have been used for different printer.
The search engine will either take you straight to the relevant page, or give you a list of matching results from which you can click on the picture of your printer.
Alternatively, you could use one of the search boxes that appear in various places on the site. These allow you to select from drop-down menus which narrow the choices available until you can see your printer listed: