The main adjustments in Lightroom are done in the DEVELOP module accessed by clicking DEVELOP in the top right of the window. The image shows the main adjustment panel, it is on the right of the Lightroom window, this is where most of the image adjustments will be done.
Take the time to go through each slider and see how it affects your image. You can undo anything you do to the image because it is a non-destructive editor, the changes are not saved until the image is exported after editing.
To reset all changes click reset at the bottom of the toolbar, to reset individual modules hold the ALT key and click on the reset option in each module.