Testosterone, once in the body, can be converted to both estrogen (via a process known as aromatization) as well as DHT. Estrogen is the main culprit for many side effects such as gyno, water retention, etc., while DHT is often blamed for hair loss and prostate enlargement. Naturally there are ways to combat this, such as using an anti-estrogenic compound along with testosterone, or even an estrogen blocker. DHT can be combated (on the scalp, to prevent hair loss) with compounds such as Ketoconazole shampoo (sold under the trade name Nizoral) as well as Finasteride (sold as Proscar in the 5mg version and as Propecia as 1mg tablets). Interestingly, this shampoo can also be used topically to combat acne on the face (or even the back if youre really flexible). Both of these methods for preventing hair loss and acne are reasonably effective. However, if you are not prone to hair loss, they may be wholly unnecessary. Male Pattern Baldness (MPB) is carried by the X chromosome, so if your mothers family boasts men with full heads of hair, then you are probably safe (unless those full heads of hair are all mullets). Naturally, as with most other steroids, your lipid profile is going to suffer a bit while on testosterone, as is your blood pressure. This, of course is nothing that can't be controlled by watching your diet and doing your cardio. If you aren't exercising, then why are you taking steroids?
To combat the aromatization of testosterone, you can simply take an aromatase inhibitor such as Arimidex. This and other Anti-estrogenic compounds are generally considered a must with testosterone doses over a gram per week. Also among side effects is increased aggression. For many, the increased aggression found from increased testosterone levels is often a bonus in the weight room.
If you really want to gain muscle mass and strength, improved endurance, enhanced recovery, testosterone will be the most important product you've ever used. Testosterone is also a relatively safe steroid to use, with some studies showing no adverse effects from 20 weeks at 600mgs/week.