If you just added a little bit of transmission fluid to the engine oil then nothing should happen to the engine.
But if you poured a quart or more of transmission fluid into the engine oil then you need to drain the engine oil completely and refill it with engine oil.
The engine oil filter should also be changed if you've had the engine running.
Having too much transmission fluid in the engine oil can cause damage to the engine bearings and possibly blow out seals in the engine.