Any time you develop a new skin lesion on your penis, you should always have them evaluated by a qualified health care professional. The type of physician you should consult about this problem is a primary care physician
such as a family doctor
or internal medicine doctor
Of course, it is impossible for any physician to diagnose a skin problem without seeing it up close. A few possibilities that it could be is (as you said) a pimple. These do occur from time to time on the penis. In addition, in grown hairs are common especially on the back half of the penis. These are seen more frequently in men who shave their pubic hair. Of course, another common cause of a pimple like bump on the penis is genital warts. These are caused by the virus human papilloma virus (HPV) and is transmitted sexually. These do not tend to give off any fluid, making this a less likely possibility in your case.
I suggest that you schedule an appointment with your primary care physician. He or she can take a close look at the pimple on your penis. From there, your doctor can determine if any additional evaluation and treatment is necessary. Good luck.