Learn how to use that product. That is a basic operation. Just look up Resize in the HELP files.
If you are using Photoshop, try Image > Image Size. Another approach is to use the Crop Tool which allows you to set the Height and Width (and Resolution).
Click Help > Photoshop Help (or press F1). Everything you want/need to know is there. If you are new to Photoshop, look at the "Photoshop getting started tutorials" on the main Help page.
Go to tv.adobe.com and look at the videos.
I've seen a number of posts in the "Photoshop for Beginners" have been ported over here to the Photoshop General Discussions forum. I wonder if novice posts here can be moved like-wise?