As a beginner swimmer you should take it easy at first and then start swimming longer as you progress in swimming.
It's recommended to swim easy for 10 minutes and then swim one or two lengths harder, rest for 10 to 15 seconds and repeat.
Then also as a beginner you can swim five laps of a fast freestyle swim, rest for 20 to 30 seconds, and then swim 10 laps of breast or backstroke.