Your penis at the age of 20 (give or take a year or two) will be the size it is going to be and is determined by genetics which is generally between 8.5cm and 10.5cm (3-4 inches) from tip to base (non erect) and will vary from approximately 12.9-15 cm (5.1-5.9 in) in length (while erect).
As far as supplements go (whether it be creams, pills, lotions, or potions) I do not suggest taking any of these supplements because they are not regulated by the FDA and could be potentially harmful to your health. I understand your concerns, but keep in mind that these products have only one action, as they separate the consumer from his/her hard earned monies. Also, another note, the manufacturers of these ‘supplements' put a lot of ‘binders' and ‘fillers' in these products and you really don't know what you are really taking which can be risky.
Altering the penis itself (either by length or girth) would include plastic surgery or penile implants which you would need to consult with a Board Certified Urologist or Board Certified Plastic Surgeon about this before making any decisions about what is best for you.