Welcome to your very own Nami guide, Diana. I made this to help you understand a little bit about Nami and supporting in general. I hope it's informative!!

I personally take these standard AP runes on almost all of my supports. They're pretty straightforward.

AP Quints are just for some good early game damage and heals.

Magic Pen Marks are there just so you can hit a little harder early game. You could also take either Hybrid Pen Marks if you think you're going to be auto-attacking a lot or just AP Marks for flat AP damage.

I find that Armor Seals are the best on Nami. If you're going to be harassing the enemy, chances are they're going to be hitting you back so you need the armor to help your survivability.

And lastly just standard Magic Resist Glyphs. You could also do a mixture of MR and Mana Regen or maybe even some Cooldown Reduction.

I like damage on my supports so I usually go 21/0/9. If you're more into the utility tree, you could go 9/0/21 or even 0/9/21 but I recommend 21/0/9 in almost all scenarios.

Things that are pretty much core on any support are Mobility boots and a Sightstone. Starting with Spellthief's Edge gives you a bit of gold income when you harass and some extra damage. Just be careful not to spam too much as Nami's abilities can get pretty mana heavy.

Next you want to start working on either your AP or Tank items. I like getting Athene's Unholy Grail for the mana regen and magic resist. Locket of the Iron Solari and Frozen Heart are always good choices if you want to go tanky. Rabadon's Deathcap or a Void Staff are ideal for a more damage oriented build.

Also, Mikael's Crucible is a pretty good support item if the enemy team has a lot of crowd control!!

Now, I always start Ebb and Flow (W) on Nami. Always. Aqua Prison (Q) may seem like a good idea but I find that Ebb and Flow (W) normally works out better.

After that, maxing Tidecaller's Blessing (E) provides a decent amount of damage and a very nice slow that's good for kiting or chasing.

I always suggest maxing Aqua Prison (Q) last, as it barely does any damage and the amount of time they are in the bubble does not increase by rank.

Flash is pretty much necessary on any support.

Exhaust is a GREAT summoner spell right now as it reduces damage dealt by the enemy champion by 50% and also lowers their Armor and Magic Resist. It helps you win fights and keep the lead. It can also be good during ganks or when an enemy champion is trying to burst you down. If you Exhaust them, the damage reduction could just allow you to stay alive.

Ignite is a more situational spell. I don't take it on support but if you feel like you're going to win the match up 100% then it's good for kill potential.

In lane, I like to assert lane dominance by being an agressive factor. That means auto-attacking the enemy bottom lane constantly, and making sure you keep wards down in vital spots to prevent ganks.

After landing a bubble, you should put your Tidecaller's Blessing on your AD Carry, unless you think they're not going to follow up, in which case just put it on yourself and attack away!

Don't be afraid to use your ultimate. Tidal Wave is a great, low-cooldown skill that provides a ton of crowd control. Oceans, spill forth!

Towards midgame, try to keep Dragon warded and the enemy jungle if you're ahead, or you're jungle if you're behind. Late game of course keep Baron warded. A support is mostly there to protect their AD Carry and ward. But don't forget to yell at your team to ward to, that's crucial, as you can only have 3 sigh wards and one vision ward on the map at a time.