(Not a Teacher)
"Some people think that using an electric heater is better than using a gas heater."
"There are many different ways to protect yourself from the cold."
"You can put on winter clothes..." (no "u" in "clothes")
"..., but the best solution is to get an electric or gas heater." ("best" needs to describe something)
"...because it's hard to deal with a huge bill at the end of the month."
"If not attached properly, the gas won't burn wholly and will lead to
thecarbon monoxide poisoning or even death. (It sounds better if you specify what kind of poisoning you're talking about.)
"If you don't leave it beside highly flammable things, of course."
"..., the electric heater has an advantage in safety." (I rearranged the sentence and added a "the")
"Thus, I agree
with the idea that people using anthat electric heaters are better than gas heaters."
MeanwhileHowever, gas heaters are more popular and convenient. The fact of the matter is that most people use gas heaters, myself included, ironically." (I rewrote these two sentences to make them sound more natural)