Should i use Viagra or medicines before intercourse?!

You are young at your age to have such problem like this.
There are underlying factors causing such erectile dysfunction, health problems or pyschological problem.

Either of the two, before taking any self medications like viagra or the like. I suggest, consult a doctor first because you never know it could be risky to yourself due to some adverse effects like a sudden increase of your blood pressure or even heart attack, contramedical symptoms that could endanger yourself. Only your doctor will know after some lab tests of your health condition as well as physical examination.
Causes of erectile dysfunction (impotence) are the following;
If one or more of these requirements are not met can cause impotence;
1.a healthy nervous system that conducts nerve impulses in the brain, spinal column, and penis,
2.healthy arteries in and near the corpora cavernosa, (penis anatomy)
3.healthy smooth muscles and fibrous tissues within the corpora cavernosa,
4.adequate levels of nitric oxide in the penis.

Furthermore, if you have heart disease, or you're diabetic (diabetes mellitus).
Too much smoking and too much alcohol, or having a history of drug abuse.

On the psychological side, it is more on fear and low self steem, or can be caused from a childhood history problem when you were child abused, depression and anxiety, there could be more on this side that a therapist or a doctor can only tell and find out the root cause of your impotence.

But the good news is, there is cure or solution to your problem, so don't hesitate to consult your doctor and seek help first before taking any self medications that just come from your friends.

http://www.medicinenet.com/impotence_ed/…

may be your stressed or have other issues see a doctor before hand you should have no problems for an erection at 25