how to play as nami?

Support aalllmmmoosstt exclusively, those that are really good with her can take her other places same as any other champ however.

Mostly practice needed, she can be a more aggressive support seeing as her main poking is also able to heal her/ally (her W), every ally hit with abilities will also gain a speed boost from her passive.

Her E is a ally auto attack empowering move that can let the ADC last hit casters under tower with 1 auto attack after a single turret shot, which makes her quite useful for adc farming, but she can also use it when she/her adc/ally is poking/engaging for a slow, speed boost, and extra damage.

Her main poking combo is W and auto attack for mana conservation poking (if you choose to do most damage and not care about mana at the time..... E, auto, W, auto, with a bubble if you choose to use it for engaging/poking)

Her ultimate should be used to engage team fights to speed up allies and knock up and slow enemies in late game, in early game it can be used to pick off high priority targets when needed.

When timed correctly you can bubble someone right as they teleport, get out of zhonyas, after they revive from a Zilean/GA revive, after zed becomes targetable after his ultimate, stuff like that, or just to CC the enemy in general.

She is support.

She is definately one of the most fun supports.

Level up her Heal first and use it to bounce off the enemy almost whever it is up to keep thier health low and keep yours topped up.

landing her bubble is a skill, practice practice practice. you really have to use it to predict where the opponent is going to move next. you can even use it to make them step towards you. So I often use it directly behind my opponents to prevent escape.

speaking of preventing escape her other ability tidecallers blessing use it on your adc when you are going in for a fight to prevent the enemy escaping but also just use it constantly to give you or your allies a speed boost from your passive. it has a low cost and low cooldown.

The wave is how you start or keep enemy teams in fights.

As for items Nami does not scale very strongly with AP. she is best with items that have abilities (you can use them using the number keys) such as mikel's crucible and frost queens claim. you use these to help you allies and set up kills for them.