Short answer to the title. You don’t (usually).
Longer answer is you can if you know what you want to grow and that there a lot of options available.
Simple lights that are pretty much set and go and more expensive and usually more powerful options.
I have Meridian 11 over my tank and I love it but it’s not necessarily the “right” light for everybody obviously. I’m a newer reefer who always has had LED lights but I’ve seen pics of older T5 and other tanks that looked incredible. Lastly do research and pick what seems like what you feel works best for your particular setup.
Longer answer is you can if you know what you want to grow and that there a lot of options available.
Simple lights that are pretty much set and go and more expensive and usually more powerful options.
I have Meridian 11 over my tank and I love it but it’s not necessarily the “right” light for everybody obviously. I’m a newer reefer who always has had LED lights but I’ve seen pics of older T5 and other tanks that looked incredible. Lastly do research and pick what seems like what you feel works best for your particular setup.

. I have no idea the PAR on this tank, but it grew stuff like mad. My "wizard" for this was a guy named Tom at a LFS in Rochester, NY: "Do what I tell you and you'll be fine." He was right!