First Time Grower - Help Me!

This is my first grow and so far so good. I am using 6 "daylight" florescent tubes, the plants have been under them for 24 hr cycle since day 1. I am thinking about starting the flowering cycle for the big guys, but I noticed that some of the bottom leaves towards the soil are starting to turn yellow and dry. I have been using a moisture meter and trying my hardest to keep it moist without it being too wet. I would appreciate any help or tips you guys have to offer. Let me know if you think i should begin flowering too please! I'm going to include the pictures of the plants. It is bushy and the leaves at the bottom worry me. Thanks

looks good...
they look like some that I got... nice...
I trim all the dead leaves away.. I heard it's best to do in morning of cycle..
It's natural for the bottom leaves to die off.

Awesome, thanks...should I change the lights from 24/0 to 18/6 before the flowering stage? or is it ok to go straight from 24/0 to 12/12? I don't want to shock the plants or anything...

Update: I am about to start flowering in a couple of days, but i was wondering what the pH of the soil should be during the flowering stage? I'll post some updated pics soon.

the PH should be in between 7.0-6.5. The light question I usually go 24/0 for the first two weeks then 18/6 till I decide to flower. hope this helps

most recent pics are included. I am about to start the budding process tomorrow. The leaves have some new, weird spots on them...if anyone has an idea to what the problem is pllleeeassseee let me know!!