You already purchased it, so ignore the people saying negative things.
Typically, using that type of system, one would start seeds in rockwool, peat pellets, or coir pellets. Then after the first set of true leaves are present, you'd move the individual plant & media to the holes in the system. For lettuces you can start seedlings in every hole, but when the plants start crowding, you'll need to take out every other one and cover the exposed hole using something like light blocking painter's or duct tape. For those six tubes take out 1, 3, 5 etc. in tubes 1, 3, 5, and then plant 2, 4, 6 in tubes 2, 4, & 6.
I'm not going to tell you to purchase a meter as the nutrients you chose won't require one. When you stitch to dry concentrates, to save money, come back and ask about Masterblend. Or any other dry concentrate available at your local hydroponics shop.
As for lights, I'd have suggested Barrina LED grow lights, but these may work if you move them significantly closer to the plants. Here is what I would have suggested for lighting for that system. One light per tube.