This stuff shouldn't be a struggle even for a mathophobe.
Assuming normal distribution.
The formula gives you the margin of error.
The z score that corresponds to a 95% CI is 1.96.
n=144, the number in your sample.
=375, the sample mean.
=20, the standard deviation.
Plug them into your formula:
You can say with 95% confidence that the mean price of all TV's(from the store population) will be between $371.73 and $378.26.
Try it with the 99% CI. See what happens to the interval. Does it widen?. Narrow?.
The z-score that corresponds to 99% is 2.575. Do you have a book with a table to look these up in?.