On the toolbar (left) near the bottom there should be 2 circles alongside eachother. Press the one on the right. Use the pencil tool on your preferred pixel size and color what you want selected. It should be a semi-transparent red. If not, check to make sure you're in Quick Mask mode (see sentence 1 and 2). Once you've colored the object in, click the left circle on the toolbar (which is also located on the bottom). Go to Select > Inverse. The object or person in which you colored should be the selected one. Next, copy it and paste it into a new image or an exiting image in which you want it to be in.

Hope that helps.