What you should do is go to see your primary care doctor
. You should never take someone else's antibiotics for multiple reasons. First, the prescription may be expired. Second, you do not necessarily know that the dose prescribe to your friend is the right dose for you. Third, you may have an allergy to the antibiotic. Finally, most cases of sinus congestion do not actually require antibiotics at all and you run the risk of creating a resistant infection if you take antibiotics when they are not needed.
Your doctor will be able to perform an examination and determine whether you have sinus congestion. Most cases of sinus congestion are actually treated with out antibiotics, using a combination of nasal decongestants, anti histamines, steroid sprays, and nasal saline washes. In more severe cases that do qualify as acute sinusitis, antibiotics may also be necessary, but your doctor will be able to advise you on that.
Your doctor will also be able to consider the alternative possibilities, which would include a cold virus infection, nasal allergies, a lung infection, asthma
, etc. Based on all of this, they will help you come up with a plan to get feeling better soon!