This is a guide for support Nami, for ranked games, but can be played in blind pick games too.

Explanation.

WHY NAMI?
Because she has heal, powerful CC's, and excellent harras beyond minions (can't be blocked by minions, like Sona's Q, or Blitz's grab)

Start with Cloth armor, buy 2 wards, and 2 hp potions.
Go cover jungle, and help pull creeps for your jungler.
Start lane, and ward important points.
Harass, harass, refresh mana, harass. ............

When you recall to base, buy Ruby stone (180 hp), basically buy Aegis of the legion piece by piece. And some wards when you need them.

Do not buy philosopher stone or other items.

RUSH AEGIS.
Aegis is best support item in game. You must have it. Sooner the better.

You do not need philosopher stone with proper runes and masteries (see below), and with clarity.

You do not need exhaust, if you rush Aegis, you will be tanky enough to survive a gank. After all what is flash for? Flash is for escape. With clarity you can always harass enemies and keep them lower on hp, which means enemy jungler does not have a good chance to come gank and kill you.

After Aegis, buy Sightstone, and place those free wards. Then go for boots (CRD boots), and kindlegem.

Usually my games ends at the point when i have Aegis, Ruby Sightstone and boots. All items after this are situational. Buy Runic bullwark if enemies have fed AP carry or lots of magic damage, in such situations rush Bullwark asap.

Best rune page for support is maximum gold gain, plus some tankiness.
5.25 gp/10s (3 quints and 9 seals)
8 armor (marks)
13 magic resist (glyphs)

http://www.mobafire.com/league-of-legends/rune-page-planner#&rune=353:3-374:9-278:9-263:9

But this can vary, various possibilities...
Can be like this
3 gp/20s (quints)
13 armor (seals)
100 mana (glyphs)
7 magic penetration (marks)

Since i start with Cloth armor and 2 wards 2 hp pots, seals can be best used for gold gain.

0 - 13 - 17

What you need is gold gain, that is highest priority.
Then mana regen, and some tankiness.
The small ward and bisquit are valuable, but not necessary.

Pickopket is not necessary. But may be good to goo deep in utility tree until pickpocket then spent all other masteries in defensive tree.
Nimble is not necessary.

Here is a good mastery page:

http://www.mobafire.com/league-of-legends/mastery-tree-planner#&tree1=0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0&tree2=0-1-4-0-2-2-0-2-0-1-0-1-0-0-0-0-0-0-0&tree3=1-0-3-0-0-1-3-0-4-0-0-1-2-0-0-1-1-0-0&v=2

Aegis of the Legion.
Wards.
Ionian boots of lucidity.
Sightstone -- > Ruby sightstone.
Kindlegem.

That is all items you need. If game goes on longer, upgrade Kindlegem into something, like Locket of the iron Solari. And upgrade Aegis into Runic Bullwark if enemies have alot of magic damage.

1. Flash
2. Clarity

Never exhaust with Nami.

If you want to pick exhaust, pick Taric, or Leona, or Soraka, or Blitzcrank, .... champs that do not need clarity, or need extra cc to kill champs from full hp.

This guide have different sense and purpose if you pick exhaust, it is CLARITY DESIGNED. :)

Start with W.
Help the jungler in the jungle, and heal him/her while he/she starts blue, wolwes, wraiths, red, etc.

In lane harass with W.
Then in second level pick Q.

On 3rd level pick either E or any other skill, preferably W again, because it is easiest to use.

Play agressively, in mid and late game you have Aegis, you can initiate fights with well landed Q followed immediatelly by R (ulti).