It will give you severe body aches and a fairly high fever. It does not usually produce gastrointestinal symptoms (nausea, diarrhea). The flu season is early this year. I work in a clinical microbiology laboratory and we have been getting requests for rapid flu tests since September. The season usually starts in January but it is really early this year.
The danger with influenza is that you can contract bacterial pneumonia on top of it. The organism that causes this, Streptococcus pneumoniae can put you into the hospital and into an ICU very quickly - in a matter of hours. This is a condition you do NOT want to mess around with. Many deaths from influenza are due to secondary bacterial pneumonia.
Here is the CDC page with some good basic information.