Click the download link and select 'Download GIMP via http
Once the file has downloaded double click on it in the download tab at the bottom of your browser (chrome) or open it from your download folder (firefox)
Follow the prompts to install.
There a number of ways to open a file in gimp.
To create an image from a blank canvas click File - New. You will be prompted with the Create a New Image dialoge You can enter the size of the blank canvas directly or you can select from various templates.
You can create an image from a variety of sources using File - Create. This menu gives you options for creating an image from a file stored in clipboard (for instance one that you copied from a web page), import from an external device such as a camera or phone or a screen shot of your computer.
Adjust color and contrast from the Colors menu.
Crop an image by clicking on the rectangular select tool in the toolbox and using it to select the cropping boundary. Go to Image - Crop to Selection.
Resize an image by going to Image - Scale Image
Zoom in and out of your image using the navigation window: View - Navigation
Add text to your image using the text tool
Double click on a tool to change its properties
Save your image: File - Save to save as the native .xcf format.
File - Export to save in another format (.png .jpg .gif etc)