Basically Nami has been one of my mains since I got league, I paid real money just to have her at the start, So id like to begin my saying Nami is a support, she is really week when it comes to being attacked and has a major advantage then most champs.
By what I mean by advantage is most champs dont have heals or speed boosts.
A way I like to place myself in team fights is bating people to come after me and set them up, realize you have to know your limits with Nami. Dont just rush into battle like Kled. Make sure you have range from the enemies but close enough you can heal your adc/top/jg/mid.

Nami's passive speeds up your team as well as yourself, place yourself at least caits auto attack range from the enemy and taunt it will make them want you more, flee to your near teammate, If the taunt isnt working the first time make sure to NOT GO BACK!
Use Her passive to get away or go into fights it really helps when you need that last hit.

Her Q is a bubble which knocks up the enemy trapped within it. the bubble uses AP and is a circle clicker, aim the bubble ahead of the person moving and you will get the bubble 70% of the time.

Her W Is her heal which can bounce up to 4 or 5 times Im not to sure lol,
Besides that her heal will lower upon each time it bounces,
The order of bouncing will be
(Team, enemy, Team, Enemy, Team)
Or the other way around.
Nami can start the spell by pressing Alt W

Her E Is a damage buff as well as a slow, her passive will allow your "Adc" to get into battle faster and auto which activates your E.
Nami can place this spell upon herself by pressing Alt E

Nami's Ultimate
A wave that knocks up enemys cought within the range of the moving knockup.
Its a wave that will go the direction you aim it in.

Nami combo
R, Buff adc (E) Q Wait for heal if adc needs it, E W E.
I would suggest Building magic resist and armor as nami, If anything have that as your runes.
Focus on your healing in battle make sure you dont let anyone die, once there is 2 people gone from a group leave the group and go with your adc, if the adc is not alive might as well support the jungle or top right?

Now here is where the smite comes into play
Nami is not a lane push champ so she needs to have that extra UMMMPHH to her.
What i mean by this is but the support item like normal yaah but use smite to smite cannon minions when they come into lane, Use smite to help your jg get Red after you leach for them, Use smite to take jg from jgler is they are useless and give it to your adc :D
Smite can also be used to take cannon minions if your adc is to scared or is just bad at last hitting.

My only tips for nami is really just stay alive and heal your team, But if you want to get funky :D lol

Build Attack speed and ap.
Youll be the apc Ability Attack speed adc LOL :P Its good because you have your own heals, your own buff, your own stuns it really is a good idea but you are really week at the start of the game farm farm farm farm, :D
And make sure you can simply get the cs but keep the enemy in lane for as long as you can dont be dumb and stay to long but go b when they b ur E and W will be available to get back to lane faster E wait 1.5 seconds then w you'll get enough mana back to have full mana when you get ot lane.