If you want to tag your friend in a post - you have a couple options. You can simply select the "tag friends" button at the top of your post specifically. Alternatively, you may add an "@" sign and begin to type in the name of your friend to include them within the post! A drop down menu will appear with the profiles that match the letters entered so far.
On a photo, you can tag your friends directly by navigating to the photo and selecting the "tag this photo." A re-sizable box will appear that you can drag over the photo and tag a specific person in.