A good read - randsinrepose.com/archives/ninety-days
Generally... observe a lot, ask questions, take notes, learn names, remember you're learning both the direct technical/work stuff and the social stuff.
If they don't brief you, ask... what is the onboarding process? are there formal induction tasks to do? Some places are really organised, others are not; if you have a disorganised place, it doesn't hurt to ask specific things like who can walk you through the code base, who is best to explain the team workflow, etc. You can create your own onboarding.
Generally when you're new, take whatever social openings present as well. If people invite you along to lunch, coffee, etc - good chance to get to know them in an informal setting.
But basically the Rands post covers it all ;)