Nami is a strong champion, who can play defensively, but is an even stronger offensive support, with strong CC especially past level 6

this rune page allows you to have extra passive gold income, while doing some damage to enemies with auto attacks. the 3 ap glyphs help later in the game, with the standard MR and armour runes giving more defensive power

standard masteries for a pre s4 support

passive, surging tides: this allows you and your adc to get in lane faster, and to chase enemies. especially strong with an ADC like vayne. cast your E on an ally to speed them up as it has the lowest mana cost
Q, aqua prison: somewhat hard to aim, but gives an incredibly strong CC on your enemies. you should only cast this on enemies when you know your adc will be able to poke them while they are stunned, as it has a very high mana cost
w, ebb and flow: you should max this first due to its ability to both heal allies and poke enemies at the same time. aim to only heal allies with it when an enemy is in range, but if you can't, don't worry about it, it has a relatively low mana cost.
E, tidecaller's blessing: this has the lowest mana cost of your abilities, cast it on the adc or any damage dealer during a teamfight, and on your adc and yourself to get better movement speed. this is pretty much nami's only way to CS, so when your adc is out of lane, cast it on yourself with the ALT key and you will be able to kill some creeps
ULT, tidal wave: this is basically an easy-to-aim Q, it stuns enemy champions for longer depending on how far it has travelled. bear in mind that this WILL activate your passive on allies, meaning casting it down a lane will both stun any enemies you are chasing and give your allies movement speed. perfect for when your jungler is ganking and the enemies are trying to escape.