Everything you know about fitness is wrong. All those lessons in nutrition at school, those info-mercials on TV selling you ab products, any diet book you have ever read, are all wrong. If you are serious about changing your body composition then you must forget all that you have learned and teach yourself. The internet is packed with a lot of good information, hopefully this is just a starting point.
My personal trainer knows what they are talking about
Most personal trainers know less than you will after reading a little bit about fitness in general. Personal Trainers go through very little actual training and only know as much as they are willing to learn. If a personal trainer ever recommends using machines before moving to free weights, walk away. If a personal trainer is not willing to show you how to deadlift, walk away.
You may find a personal trainer who knows what they're talking about. You'll have found one in a million, they do exist, but they are hard to find. If you get a free session with a personal trainer and they are not willing to show you particular excersizes, or contradict what you have taught yourself about nutrition, excersize or general fitness, walk away, they'll be wasting your time and if you start paying them, sucking away your money.
Machines are better than free weights
Almost without exception, this is not true. Machines are not built for the individual, they are built to cater to one percieved 'normal' body shape. If you are slightly taller, slightly shorter, female, slightly longer arms, slightly longer legs and so forth the machine will not fit your body. As a result it will force your body to move unnaturally, which is often called 'bad form' in freeweights. This leads to injury and poor results.
Machines also remove stalbilizer muscles from the equation, while you may want big broad pectorials, you're forgetting all the other muscles that surround these areas and help support and move your body during resistance training. Machines rarely engage stabilizer muscles because the machine guides the path of the excersize.
Free weights are dangerous
Again, this is a complete fallacy pushed on consumers to sell overpriced gym equipment. Free weights are far safer than machines, they allow you to follow a natural path which maintains perfect form, reducing risk of injury. People will argue that machines are easy to get right and less intimidating. Sorry folks, weightlifting is not about being easy. This is supposed to be hard work, you're supposed to treat the iron with respect. If it's not hard work, it's not working.
If you lift with good form and progress in a safe manner, free weights will give you years of injury free workouts. There is no substitute.
Lifting weights will make women bulky
Sorry ladies, unless you take testosterone you will never be 'bulky'. Men and women are not created equal, that's not sexism, that's the truth. You need testosterone to build muscles, women produce a 10th of the test that men produce. Without test, you will struggle to build muscle. Those 'ladies' you see competing in body building competitions are so full of test that they're basically men. You will never bulk up, no matter how hard you try. I promise.
I'm skinny and skinny is healthy
I remember seeing a body building shirt that said "I see skinny people" which is a nice way to say that just because you are skinny, does not mean you are healthy. Your body fat percentage is linked to your metabolism and diet. If you are skinny, that just means you are not eating enough to maintain a healthy body weight, regardless of what you are shovelling into your mouth.
You might hear someone say "I eat crap all day, never excersize and never gain weight haha" which means they're not consuming enough calories, but they're also not consuming enough nutrients to live. You'll find they're probably constantly sick, suffer from bowel issues and poor nutrition.
Your BMI is not always an indicator of health.
I'm too old to lift weights
There is proof that shows resistance training in elderly patients helped fight the effects of osteoporosis. They're not talking about grandma benching 200 pounds, they're talking about small amounts of weight and compound excersizes resulting in an increase in bone mineral density (BMD). The opposite happens to people in space, the gravity bearing bones experience a drop in density while in zero gravity.
I'm too poor to join a gym
Not all gyms are multi-francise mega gyms with saunas and massage tables, you'll find powerlifting old school gyms around if you look hard enough. Your university might have a gym, or your local TAFE. If you've got a PCYC or YMCA nearby, they'll have a gym too. A lot of these places will charge a very small annual fee, or sometimes charge per visit.
Before you sign a gym contract, read it carefully, it is no secret that mega-gyms survive on ripping off their customers. You will be expected to carry out the full term of the contract, you will pay through the nose for cancelling early, you will pay through the nose even after your contract finishes. Attend with caution.
Alternatively, if you have space, you can train at home. One of the best things about resistance training is that everyone gives up. The trading post, ebay and garage sales are full of second hand gym equipment that no one can get rid off. Pick yourself up an olympic bar, some weights and a powercage with a bench and you have all the equipment you need to get started, sometimes for free!
Gyms are full of roid freaks
Not true. Gyms are full of idiots, that's for certain, but roid freaks are few are far between. The biggest guys started somewhere, as much as genetics are involved, they all started skinny or fat or weak. Most people at gyms have their own goals on their mind, if you keep out of other people's way, re-rack your weights and don't make obnoxious noises, you'll fit in fine.
I don't want to look like a condom full of walnuts
Sorry guys, that's genetic and steroids. Natural body builders look like the guys on the cover of Men's Health except they have a lower body fat percentage, professional body builders look like a condom full of walnuts. To reach that elite level of professional body building takes decades of dedication, many steroid cycles and the genetics of a superhero. Much like women can't bulk up and get 'man arms' without testosterone, you'll never accidently look like Arnold or Ronnie by lifting weights. Those guys eat 10,000 calories a day and are walking chemistry laboratories. When they're in competition form, their lack of body fat would kill a person if they tried to maintain it for more than a few hours. It's simply unachievable except for a select few, you are not one of them.