Check System Preferences Mouse pane (or Trackpad pane). For the Mouse pane, make sure the Primary mouse button is set for the one on the left.
The Control key toggles the "right-click" with the primary button, when the Control key is pressed down while clicking the primary mouse button (or on a Magic Trackpad). So make sure you are not pressing the Control key inadvertently.
Also, if you are using a Magic Trackpad (separate or on a MacBook), the botton portion (along the edge closest to you) is sensitive to clicking on the right side versus left side. The rest of the trackpad is not. So, if you are clicking near that edge closest to you, a click on the right side will be a right-click. It is designed this way, because some people use their thumb along that edge to click (not their "pointing" finger); on old-school trackpads, physical buttons were located there. So, you can still point with the index finger and click with the thumb on the right or left "button." You should be able to adjust this behavior in System Preferences Trackpad pane.