Markusgomez,
Before you start thinking about viagra, I think you need to determine if other things are inhibiting your about to get an erection. At 17 you should have no problem getting a rock hard erection. Perhaps there could be stress or other worries going on in your mind and this would inhibit you ability to perform sexually. But, if you are really worried then it would be a good idea to take a trip to your doctor for a little chat and perhaps even an examination of your penis and testicles.

Wendy