Photoshop is only available via subscription via the Adobe Creative Cloud, which can be paid monthly or annually. There are a number of plans to choose from.
Until recently, the cheapest way to download Photoshop was a single-app subscription. However, at time of writing, Adobe seems to have removed this option. So the cheapest plan right now is the Creative Cloud Photography plan. This costs $9.99/month and includes access to Photoshop, Lightroom, 20GB of cloud storage, Adobe Portfolio and Adobe Fonts. If you need more storage, then $19.99/month will increase your storage to 100GB.
If you want to access other Adobe desktop apps as well as Photoshop, such as Illustrator, After Effects and Premiere Pro, you'll need the All-apps plan. This is the most expensive way to download Photoshop. But along with everything in the Photography Plan (see above), you'll also get access to 20+ desktop and mobile apps including Illustrator, InDesign, and Premiere Pro.
Finally, if you want to work on your iPad, Adobe offers the Design Mobile Bundle which saves you 50 per cent on the collective price of five mobile apps: Photoshop, Illustrator, Fresco, Spark Post, and the Creative Cloud app.