You should ALWAYS buy 1 or 2 wards until you are finished with your gear.

BUT it's not always possible, so here we go:

Let's split by lane:

JUNGLE

Buy 1 or 2 when you get in the base after your first walk in the jungle. keep trying 1 or 2 every time you get back in the base

TOP

Try buy 1 at the beginner of the match or as soon as possible, recall and shop 1 or 2. Keep at least the river warded.

MID

Try buy 1 at the beginner of the match or as soon as possible, recall and shop 1 or 2. Combine with your jungle, so he can ward one part of the river and you do the other.

BOTTOM

If you are with a support, you can focus more in your gear. But you need buy some too. So when you have a spare of money, buy a couple or it and help warding dragon and river.

Here a great link for warding map

You should ALWAYS buy 1 or 2 wards until you are finished with your gear.

BUT it's not always possible, so here we go:

Let's split by lane:

JUNGLE

Buy 1 or 2 when you get in the base after your first walk in the jungle. keep trying 1 or 2 every time you get back in the base

TOP

Try buy 1 at the beginner of the match or as soon as possible, recall and shop 1 or 2. Keep at least the river warded.

MID

Try buy 1 at the beginner of the match or as soon as possible, recall and shop 1 or 2. Combine with your jungle, so he can ward one part of the river and you do the other.

BOTTOM

If you are with a support, you can focus more in your gear. But you need buy some too. So when you have a spare of money, buy a couple or it and help warding dragon and river.

Here a great link for warding map