Take a look at this site. This is very helpful. http://www.vocabulix.com/conjugation/German-Verbs.html. Type any word 'schwimme' and click 'search.' And the click 'Show All' for 'Present.' You will find what to use and when.
Scroll to the bottom of this page and read the tips (especially the last one): https://www.duolingo.com/skill/de/Basics-1
These tips pages are only available on the web version for now (not the app), but they are very helpful. I recommend checking to see if there are tips every time you start a new section.