There are two (main) ways to do it:
Way #1
1. Select your text layer.
2. On the bottom left of the layers window (where all the layers are listed), there is a button with an "f" on it. Click this and select "Stroke..."
(alternatively, go to the "Layers" menu, and go "Layer Style > Stroke...")
3. Play with the size, pattern and colour of the outline there. Click OK!
Way #2
1. Select your text layer.
2. Right-click it, and select "Rasterize Layer" (note: if you do this, you won't be able to go back and edit your text).
3. Go to the "Edit" Menu and select "Stroke..."
4. Select your colour and stroke size and click OK
The first way is somewhat preferable, because it is more flexible. You can go back later and change your text, or the colour and size of the stroke... which is good if you're experimenting with different effects