Taking supplements really depend on your goals. whether you want weight loss, get toned, build muscle, put on weight, etc. there are tons of products on the market, some have multi-million dollar R&D divisions, and some are owned by pharmaceutical companies.
While many companies are solid companies, there are tons of companies that are just me too and they have real no purpose because their products are made by an outside company that makes the same product for 5 other companies or their product is a copy of another, and they are just trying to make money without putting in the work or having any knowledge.
I have been an athlete all my life and have taken supplements for most of that time. So I have been around the block a few times, you might say.
Every supplement you take should have a purpose, you take vitamins because you don't get enough in your diet, whether you have a balanced diet or not just because the food is processed so much their are not enough vitamins and minerals to meet your daily requirement.
The same goes with sports supplements or fat loss. The results you want and the results you get depend on the supplement and your approach.
Fat burners make a lot of people jittery because they have stimulants, however some don't.
As far as building muscle the supplements are there to do just that supplement what your body naturally produces. however under stress (workouts, exercise, sports) your body can not make enough of the required compound, nutrient, etc to provide your muscles or help rebuild them after.
If you want more information just ask, this is a short overview.
However one suggestion is, I have never heard of people just supplementing with Carnitine ( or did you mean creatine?). Also I wouldn't buy your products from the gym, just because you can get the same product or better for cheaper from other retailers, usually a lot cheaper.
I buy my supplements online, for the very reason I stated before cheaper and they are the same product or better. I mean like 30% - 50% cheaper, not just a little bit. For example I have been buying my products from, Fuel 4 Life, the last year or so, and a protein I buy is $20 - $30 cheaper from there than say GNC or vitamin shoppe or your local gym.
If you have any questions just ask.