Add me and I can show you if we lane together. Basically on Nami, poke as much as you can with AA/W/AA. max W first because it's not a skillshot and you win trades most of the time bc it heals you. Depending on how were doing, I max Q or E next. If I'm playing with an aggressive ADC, I tend to max E for the extra damage and slow (great for chases with Nami's speed boost passive).
Typically you want to ult as an engage or disengage. Fighting in the jungle is more advantageous (ofc depends on team comps though) because there's less space to dodge Nami's ult. Ult sets you up for a Q followup because it slows. So a trick is if you ult or someone gets hit by an E-buffed AA, Q them because it's easier to hit the Q.
For the Leona scenario, I typically W my ADC, ignore Leona, and bubble the ADC because THEIR positioning is predictable if they follow up. Then I E my ADC and W again when I can. Leona doesn't do damage, she just stuns and roots, so your ADC should be aiming at theirs (Le9na needs to run bc she used all her skills on your ADC, so free damage for you).
I start with a spellthief, 2 HP, 2 mp pots. Then sightstone/boots. Recently I take a Tear if were stomping lane hard because Nami's pretty Mana hungry. It's like other support builds... depends on the situation. Randuins or frozen heart, banner of command is good on Nami, lich bane if you're super ahead/Yolo mode, locket or mikaels if you need it