Well, to start you off, go to the adoption page and adopt the maximum number of each pet currently available. If you have real cash to spend, buy some CS dollars and hit up the store, or hold on to some.
The pets grow up throughout the month, and then new ones will come out on the 1st.
Take a look through the archive. Add your favorite pets to your wishlist. Using the current moth's pets that you do not care for, try to trade for pets a little older. Keep doing this until you are able to trade for uncommons and rares, and then you can try to trade for 2008 or 2009 pets (which are worth more in CS terms).
Browse the forums to find things that interest you, and make friends with whom you can trade with in the future.
Study the posts here -
viewtopic.php?f=20&t=202027 to see what each pet is worth.
That should start you out nice. And try not to send ridiculous trades, like a bunch of commons for and uncommon or rare, unless the person states otherwise XD