Ensure you have the latest version of Photos. Tap the Menu button (the three vertical lines) in the upper left-hand corner. Then tap Collections, then use the drop-down to choose Albums. Click the plus sign on the upper right-hand side, then choose to Create New Album. Tap the photos you want in the album (you should see a check mark on the photo once selected). Tap Create in the upper right-hand corner. Name the album and press the Done key on the keyboard.