Boosts are not very useful early in the game. The Revive is the best boost to purchase, for 750 coins.
Combos
Destroy three similar obstacles (squirrels, birds or ninjas) in a row to get an invincibility bonus. To do this, continue to attack the ones that you already have hit until you get to three and jump over the other types. The easiest to obtain is the squirrel combo.
Upgrades
The Shield is the most important upgrade, as it basically is an extra life. Spend your coins and upgrade it to least level 2 as soon as possible. Super Size is a good upgrade because it is similar to the Shield, but it wears off over time.