Hi Liv,
Just a heads up, the post below & the links I put in here talk about sexual function in a very frank way and contain NSFW images that some people may not wish to view. Please skip this post if that's you! I worried a lot about side-effects, so I did a lot of research--that being said, I am not a doc & this is no substitute for medical advice.
I got a sheet from my doc on this before I started, and it laid out certain risks of estrogen HRT. I was glad to be aware of these risks when making my decision. This post from Silly Trans Woman covers pretty much the same info as the sheet my doctor gave me:
http://sillytranswoman.blogspot.com/2011/03/risks-associated-with-hormone-use.htmlHere are some common side effects and some lifestyle changes:
side effect: increased risk of blood clots (which can cause other problems)
lifestyle change: try not to sit for long periods of time without getting up; quit smoking(!!!); eat a healthy diet; get 90 minutes of exercise a week; inform all of your doctors about your medication so that they can help manage your other meds and help you decide if you need to stop your HRT, say, for a couple of weeks before a procedure; my doctor recommends switching to injections or patches after age 35 because some studies show that these delivery methods present less of a clotting risk than pills (I am sure KayXo could cite the studies--she is an incredibly smart person who has really done her homework on HRT!)
side effect: increased risk of developing a tumor of the pituitary gland called a prolactinoma
lifestyle change: make sure your doctor is monitoring your prolactin levels; I'm not aware of any preventive measures you can take. This one may seem scary, but it's also a side effect of certain medications for depression and psychiatric disorders, and it also occurs naturally in some people. The word "tumor" is scary, but a prolactinoma is benign tumor (aka, it's not cancerous). If you do develp a prolactinoma, it is usually treatable with medication and/or surgery.
side effect: decreased sexual functioning (loss of libido, inability to maintain an erection, decreased volume of ejaculation)
lifestyle change: make sure to get enough rest and exercise, communicate with your partner/s, set aside time to explore your body's changing erotic landscape on your own; consider sexual techniques that allow for different experiences of and relationship to your body (see this NSFW site
http://early2bed.com/2014/02/05/trans-women-sex-awesome/ ); if you would like to penetrate your partner and are unable, consider using a strap-on device; avoid drugs and alcohol, which tend to impair sexual function on their own.
side effect: infertility
lifestyle change: bank sperm
side effect: decreased testicular volume, penis size
lifestyle change: if you could figure out how to reliably increase the size of the male member, you would be a billionaire!

Many trans women do not find this change distressing. If you do, explore techniques above that might help you work around self-consciousness about your genitals. You could also pursue strap-ons or other toys, or engage a therapist to help you work through these issues.
side effect: mood changes (depression, anxiety, suicidal thoughts)
lifestyle changes: start working with a therapist; keep a mood journal to track any changes (it can be as simple as a smiley face or frowny face written on a normal calendar); make sure to get plenty of rest and exercise; try to eliminate stress from your life, where possible; let your doctor and therapist know if you experience these symptoms--I know someone who developed a panic disorder on HRT, so she added an anti-depressant to her regimen, and that really helped her and she was able to continue her HRT.
side effect: possible increased risk of gall stones (this one is on Silly Trans Woman but wasn't on my doc's fact sheet)
lifestyle change: eat a sensible diet, more here
http://www.webmd.com/digestive-disorders/understanding-gallstones-preventionside effect: possible increased risk of breast cancer (this is actually a controversial one, again if you must have the studies, KayXo could probably point you toward them)
lifestyle change: follow the recommended schedule for mammograms for a woman of your age; use self-exams between mammograms to monitor your breasts, here is a guide:
http://www.nationalbreastcancer.org/breast-self-exam (note that if you follow this guide, it may scare you because it warns about lumps under your nipples, and at the beginning of your HRT, you will develop some hard, painful bumps under your nipples--that is a normal development called a breast bud; your doc can examine you if you are concerned)
Most of this stuff boils down to common sense--eat a healthy diet, exercise, and pursue HRT through a doctor who can help you monitor your risk. Almost all medications have a host of common--and uncommon--side effects of various degrees of seriousness, and estrogen is no different. Medical experts agree that estrogen HRT is safe when prescribed and monitored by a doctor, so your friend is definitely being unhelpful by calling HRT "dangerous." Many people take on the risk of serious side effects to treat non-life-threatening conditions like sexual dysfunction, acne, hair loss, etc. You rightly point out that for most trans people, the benefits of HRT far outweigh the risks.
Some steps docs take to lower your risks are:
1. To screen you for conditions that could make HRT more dangerous. You should talk with your doctor about any conditions you have (circulatory, heart, liver, kidney, & mental illness come to mind) or family history that might increase these risks to you.
2. Side effects generally increase with dosage, so your doctor may try to use the lowest effective dose. You should follow the dosage prescribed by your doctor and then communicate with your doc if you feel that your progress is not sufficient. Be aware that your mileage may vary with HRT--your results won't necessarily match others', and it takes time.
3. Doctors also order bloodwork periodically while you are on HRT so that they can make sure you are not experiencing certain serious side-effects.
Finally, if your friend says hormones are "dangerous" but "natural" methods aren't, you should know that taking a "natural supplement" that is supposed to mimic HRT is not necessarily safe at all, as most supplements are not evaluated by the FDA. Talk to your doctor about any supplements, herbal medicines, or diet changes you plan to pursue alongside or in lieu of HRT. HRT often includes a testosterone blocker as well as estrogen, which has its own side-effects depending on which medicine you are prescribed.
Hope this helps!,
Patti