Sometimes with spellwork, timing is everything. Just like you'd use lavender in a sleeping pillow or carry a moonstone for luck, there are times that are best for doing certain work, like say, fixing a friendship, starting a new activity or saying goodbye. Looking at the moon's phases is one way to determine timing for a spell.
Let's say you're trying out for a team. In addition to practice (NEVER substitute magic for hard work and practice!), you may want to do a spell to help you stay focused at the new moon—a time of beginnings. Like the earth where seeds are planted, the new moon is dark and rich timing-wise for starting things. As she waxes (grows bigger), so will your intentions.
On the other hand, start a spell at the full moon to say bye-bye to nail biting or hair chewing—or any other bad habit you'd like to break. As she wanes (shrinks) so will the habit you want to toss.
Here's a spell to do at the full moon—all you'll need is a dark bowl, some water, and a full moon.

Fill your bowl with water and go outside. Holding the bowl with both hands, walk around until you catch the reflection of the moon in the water. Important! Don't do anything dangerous—like stretching out of windows or balconies to do this—save this work for when you can walk around safely (like in a park—and because this is a nighttime activity, make sure an adult is with you!)
When you've caught the moon, stand still and look at the reflection, picturing the energy of the moon going into the water.
There's lots of things that you can do with your moon water. You can use it during ritual to bring the energy of the moon to your celebration. You can use it to bless and charge tools, or add it to pouches and pillows you make to add moon magic to your work.
You can even use it (sparingly—a little goes a long way!) in recipes. Often, sabbat rituals involve sharing food. Add three drops of your moon water to batter for breads, cookies, or my personal favorite--brownies and concentrate on intentions for good health while you mix and bake. Be sure to let the folks you give them to know that you're sharing magical food—and what your intentions are.
Have a magical Lammas and remember—harm none!