In Nami's early game, it is important to be agressive, unless you have a very defensive AD carry, Nami is very good at poking/hurrasing, but she runs out of mana quickly.
Nami has Spellthiefs Edge, which means that everytime she hits a basic attack, she will gain gold. So try basic attacking your opponents is possible, so you can get extra gold from the start.
When playing Nami, try staying out of the center of the team fight. Make sure to heal everyone as much as possible, and try farming a bit for the gold.