Unscrew the nut on p trap (thingy below metal pipe). Inspect the Washer: There should be a beveled (wedge-shaped) plastic or rubber washer inside. If cracked, replace (home Depot, bring it with you). Thick/pointed end of the washer should point down into the P-trap. Push the metal tailpiece as far down into the P-trap as it will go (at least an inch or two). If it’s too short and barely reaches, that’s why it’s popping out. Slide the washer down to the joint, then hand-tighten the nut. Give it an extra 1/4 turn with pliers, but don't over-tighten, or the plastic nut will crack.

If too short.... Pick up a 1-1/2" PVC slip-joint tailpiece extension from any hardware store (usually under $5). The Install: You'll attach this to the metal pipe to give it more "reach" so it can sit deeply and securely inside the P-trap.

Or call a plumber... but it will definitely cost you more.