This post doesn't contradict Bibblebub's suggestion. I'm just trying to fill in a bit of a detail in case you're a complete novice with Photoshop:
1. Use the lasso tool to draw around the area you wish to isolate.
2. Right-click and select 'feather' in order to soften the edge. Try 5 or 10 pixels as your initial setting. (That stage isn't totally necessary but it usually improves results).
3. Right-click and choose 'Select inverse'.
4. Use the fill tool to cover the 'outside' area, which is currently selected, with your choice of foreground colour from the palette.
Chris