You can start anywhere you want! I often recommend starting with Star Trek: The Next Generation, since it’s aged a little better than the original series. You might prefer to jump ahead to season 2 or 3 to get to the really good stuff, but even season 1 is worth watching.

Up until Enterprise season 3 it’s pretty much all episodic (or in DS9’s case, mostly episodic with a subset of the episodes forming a series-long story arc), so you can pick a random episode with a cool-sounding description and start there if you want. That’s how I got into Trek, just picking random TNG and Voyager episodes.