Try to stay behind and close to the boat in front of you to earn drafting points. After drafting long enough, a light blue tint will appear and you will get a speed boost. Use drafting boosts to maintain your speed, especially when out of nitro.
Nitro
Nitro boosting will allow you to go through any of the other boats, including the police. To get past an opponent easily, use nitro to wreck their boat. Nitro does not have to be fully charged to be used.
Sponsors
You will obtain your first sponsor early in the Miami set of levels. They will initially only offer you about $40 per hour. However you can bargain them. This first bargain option will probably have a 100% chance of success, so use it. Afterwards, your chance of a successful bargain will drop. In order to get more sponsor offers, improve your lifestyle by buying items.
Upgrades
You will sometimes have a choice between collecting a nitro boost or a ship part while going off a ramp. If you have enough of a lead and the time to spare, try to get ship parts. They are used to upgrade your boats, and you will need to upgrade the starting boat immediately to remain competitive.