1. Learn by doing
The best way to learn how to use a software like Photoshop is to actually start using it right now. Buy a copy of the software, install it and dig into it right away. If you feel overwhelmed and don't know where to start, just open an image with photoshop and play around with the tools that you see. Just use them randomly for a while and browse through the menus, to get a feeling of what happens in the software. Now of course that won't help you much, so the next thing to do is to read the official manual from Adobe by clicking on the Help button. You will be redirected to the online help, in which topics are sorted by category. You can simply start reading and try to apply what you learn as you go.
2. Find resources online
This is where you will find the most resources. There are literally thousands of websites teaching how to use photoshop in one way or another. I will list some of my favorite websites:
Tutorials:
As I mentioned before, the best way to learn is by doing. That's why tutorials are so popular for photoshop. It is much easier to learn how to use photoshop by following a tutorial on one particular topic, step by step, than reading a whole book and then forgetting the first chapter before you even start. Here are some good websites for tutorials:
PSD Tutsplus
By far the best resource for photoshop tutorials. The site has been online for years, and they publish new tutorials every single day. There is also a premium membership for advanced tutorials and special downloads.
Layers Magazine
The website is full of tutorials and articles about the Adobe products. This is the link to the photoshop tutorials.
Smashing Magazine
is another website offering wonderful tutorials.
Photoshop Lady
bookmarks tutorials from the web.
SpoonGraphics
is one of my favorite website, with lots of tutorials and regular roundups for design inspiration.