The Crop and Resize tools are in the Edits menu on the left.
This video will walk you through how to use each of the tools and explain what the difference is between the two.
To crop an image:
Click the Edits tab on the left, then choose Crop from the Edits menu.
Use the corner handles of the bounding box to select the area you want to crop.
Alternately, you can choose a preset size from the proportions drop-down, or enter your own dimensions.
Check the Scale photo box to keep your bounding box the same relative size.
To resize an image:
Click the Edits tab on the left, then choose Resize from the Edits menu.
Enter the desired dimensions in pixels in the Width and Height boxes.
Alternately, you can check the Use percentages box to change the size of your image.
To unlock the current proportions, click the chain link button.
To read more about the difference between Crop and Resize see: