Norvy prefers white it seems, while I tend to stick with the green beasties. Green offers many low cost creatures to block with such as Wall of Wood, Bears, and Armodon. Your larger creatures should drop quickly because of the land advantage cards such as Civic Wayfinder and Rampant Growth, so you shouldn't have a problem fending off their advances.
The Wayfinder and Rampant Growth also help thin your deck out to find the cards your looking for: Wurm's Tooth and Natural Spring. Your base deck comes with two Natural Spring, and by beating the first five levels of campaign you also unlock two Wurm's Tooth.
Wurm's Tooth out early on (especially against a green deck) is an awesome life advantage. It gives you one life for every green spell played by anybody. This will be your slow but steady life gain during the game.
Natural Spring is a one hitter quitter. 5 mana for 8 life. If you've got the Tooth in play you get nine instead.
Now just like Norvy said, play defensively. Save your Giant Growth's to pump up your blockers, and don't bother attacking. Eventually you're going to pull Duskdale Wurm, a 7/7 trample. You're going to want to put a Blanchwood Armor on him to pump him up to 14/14 (or higher) and intimidate your opponent into not attacking. This will allow you time to play more green spells to gain life with the Tooth. It should only take around 10-15 turns with the Tooth out to get up to 30 life, and most of the time you'll pull at least one if not both of the Natural Springs to help bump it up that much more.