Even if you become a eunich, your body still produces testosterone. The adrenal glands produce about 10% of testosterone of your body. It also produces DHT (at the zona reticularis).
Besides that, as other people stated, you'll become more feminine.
Your muscle mass will decrease, your fat tissue will increase, you'll completely lose your libido. Your energy level will decrease.
And also, you won't lose body hair, mustache/beard, nor will any hair you already lost on your head reappear.
Besides, if you are so afraid of hairloss to go with castration, there are drugs your doctor can prescribe you that will cut off all testosterone creation, allowing you to experience the effects of castration, that are completely reversible once you get off the drugs.