>First off, are there any Canadian security rules that I should be aware of?

No - You're flying into the USA, so the USA rules will for all intents and purposes apply.

>will I clear customs in YVR?

Yes. The first thing you need to do upon arrival at the airport is fill out a US Customs declaration card. You'll see racks of them all over the place at US Departures.

>If yes, how early do I need to arrive at the airport beforehand?

Depends on the time of day, but I usually allow 2.5 hours for US-bound flights, as there's 3 queues:

- check in
- US customs
- security

If you've got a checked bag you drop that off between US customs and security.

Note that if you're on a commuter plane between YVR And SEA you'll have a bit of a walk to your gate.