I have a heater that has 1500 watts in my room and a dryer in the garage. When we use it at the same time, we get a black out. When I go outside to check the breaker, the switch on the breaker is off instead of on. The breaker amp is on 20 right now. I'm wondering what is the maximum amp I can use to get rid of the black out problem.

You need to separate them. They should not be on the same circuit.
You cannot just use a larger breaker. The breaker is sized to the wire on the circuit.

Stan is right about sizing the breaker to the wire. However, is this a 120 Volt dryer? Most are 240VAC. What type of heater? Is it 240 or 120VAC? If you are tripping a single 20 amp breaker and the dryer is 240VAC, something is wired improperly.

Guessing that the brancg circuit is a multioutlet 120 Vac - 20 amp circuit, which presupposes the dryer is a 120 Volt / Gas appliance.

The heater, at 1500 Watts / 120 Volts needs 12.5 amps for itself.

You haven't given us the required amperage for the dryer.

However based on a few things I can "Extrapolate (guess)" that you are trying to draw too much amperage.

A 20 amp multioulet branch circuit is limited to 16 amps. The heater all by its lonesome is using 12.5 of that amount.

But more information is needed before I can give other than a guess.