You might want to try the ellipse tool:
If you can't find it, it's probably hidden below the Rectangle tool. Press and hold the rectangle to have a series of other tools pop up.
- Draw a circle in your file by dragging with the tool while holding
Shift. This will most probably yield a coloured circle
without an outline.
- Seek out your Layers panel. If you can't find it, check
Layers or press f7 to make sure it's present.
- In the layers palette, you'll see a layer named 'Shape 1'. Click it.
- At the top of the layers panel, look for the 'Fill' precentage and set it to 0%:
In this way, you tell Photoshop to hide the layer content while still retaining the Layer style we're going to add.
- Now double-click on the 'Shape 1' layer in the layers panel, anywhere but on the layer's name or thumbnail to pop up the 'Layer Style' dialog box.
- Click the word 'Stroke'. Don't just click the checkmark, click the word.
- Set size, position and colour for your stroke and play away. Commit with 'OK'.
You can change the Shape layer's size and position, and the outline will persist.