I planted a Lettuce in my veg garden- when and how do I pick it?

https://www.almanac.com/plant/lettuce

My dad always broke it off at ground level and it would come back and reproduce. You didn't mention what kind of lettuce but he always planted leaf lettuce.

remove outer leaves from the bottom -- it will continue to produce until your weather gets too hot and it bolt ( go to seed ) , but you can get a few more days/week by cutting off the flowering/seed stalk that comes straight up out of the middle.

I use a scissors to cut them at ground level... you will then be rewarded with another crop!

Stop by the place where you bought the plants or seed and read the instructions, or if you remember the variety you planted, look it up by asking on line, "when to pick ABC lettuce!"