If you are going to build the aggressive build go base for first time when you have 1250G and for second time when you have 1100G

If you are going to build the passive build go base for first time when you have 1000G

Nami's passive gives speed to the allies hitted by her abilities, even R and Q, if your W and your E are on cooldown you can use Q on allies to give them speed.

Nami's Q is so slow and it's easy to dodge, try to cast it on slowed or rooted enemies, or slow them with E + W, E + aa or using E on an ally.

In early, Nami's W is weaker every time it hits a champion, until it's growth is positive cast it on the enemy you are focusing or the ally with lowest heal first, in late, it becomes stronger every time it bounces, try to cast it so it hits your main objective the last.

Nami's R gives your allies the double of the speed of your passive, what can be useful while chasing enemies during a gank of your jungler.

If the enemy botlane has a better early than yours, use the passive build, otherwise use the aggressive build instead.