There are a couple of possibilities to keep the lower end of your shirt below the upper limit belt-line of your trousers.
Many of the solutions presented have been focussed on the fastening with pins, clips, or adhesive tapes or by purchasing increased lengths of the lower end of the shirt. Some techniques appear to be severe.
Alternately, you may be wearing your trouser belt-line too low for your normal daily routine and your body shape and proportions.
The hack for your situation until you can affect a permanent solution (proper-fitting clothing) is to wear your pants higher around your waistline. Wear your pants at a height sufficient to keep your shirt tails in place below your waistline. You might even want to wear suspenders to deal with this situation. Suspenders allow you to wear slightly looser pants that will more easily ride up and down as you lift and lower your arms. They are more comfortable than wearing a belt for many.
In the future, if you feel your trouser belt finding its normal position on your body lower than will accommodate your present shirt-tail length, seek longer shirts that will suit your body proportions and belt-line trouser length.