I like to make a very clear difference between strength and energy in one's voice. Strength can sometimes lead us to the false conclusion that we need to push or shove, or otherwise help our voices get out of our bodies. This is not so.
The true home of both strength and energy for the voice is in the breath support, the muscles around your middle, both front and back. you will hear it called belly breathing or diaphragmatic breathing as well. When you learn to guide every tone from the center of your body, the mechanism in your throat will be left in peace enough to go about doing its job properly and easily.
so learn about your breath support system first.
the next step comes in becoming aware of how you use and shape your throat, jaw, mouth, and lips to create and modulate the sounds produced by your vocal cords.
this is a good time to mention lessons: if you not taking them now, you ought to consider doing so soon. A chorus director, whether church or school can be the first step in that direction.
Other purely physical things you can do to strengthen your abdominal muscles are leg lifts, and laps in the pool. A sports trainer can answer abs questions.
Best wishes and keep on singing