Do not place this young plant in direct sunlight. Gradually introduce it to more sun over the coarse of a few weeks. ALWAYS PROTECT FROM TEMPS BELOW 50! Bougainvillea are easily damaged by cool weather!
Bougainvillea's like moist well drained soil. Don't let it get bone dry but don't keep it soaked either. Just evenly moist & well drained. Reduce water in winter.
You will sometimes have yellowing leaves this is normal as the leaves go through natural leaf death.