You need to floss between all of your teeth. I would even suggest that the teeth in the back are more important to floss, because there is more likely to be debris that gets lodged in back there.
Part of the reason why flossing is so important is because recent studies have shown that infections that enter/start in the mouth can lead to heart disease, more so than an infection that you might get from a cut on your finger for example. You can read about it in this article by The American Heart Association When you brush and floss regularly, you are getting rid of debris that can have bacteria that can cause an infection to enter your blood stream. Also, the debris that’s left in the back of your mouth, between your teeth can start to rot and cause bad breath (that you may not even notice, but other people will).
Brushing and flossing are just part of regular, routine hygiene. Did they not teach that in your school? Do you regularly visit the dentist?
You should brush after you have breakfast (and maybe do a swig of mouthwash) but I think it’s ok to floss just once a day (at night before you go to bed, after brushing, then use a swig of mouthwash after flossing) That should do the trick.
If you are having a hard time getting floss between your back teeth, because they are tightly spaced, you may have to experiment with different brands of floss. My teeth are pretty tight and I found that Oral B’s brand of floss works best for me. You might also try Glide brand or brands that are described as “tape” rather than just floss. The tape types are usually flatter and thinner and fit between tight spaces better. They also have these disposable floss picks that make flossing in the back a little easier.