Find a mentor. If you have friends (who backpack), go backpacking with them. Observe their gear. Borrow items of interest when that is possible and try them out. Pester people who seem to know what they are doing or who are carrying a piece of gear you are considering buying. Ask questions.
Every gear choice involves trade-offs. We are balancing cost, durability, weight, performance, availability, and sometimes even appearance. One person may want to emphasize low cost while another wants the lightest thing possible and is willing and able to pay for it.
There are almost always several alternatives that are almost equally good for a given item at a given price point. Looking for the "best" pack or sleeping bag is a futile exercise. I don't hesitate to make recommendations though. My recommendations are unlikely to steer you wrong, but there always seem to be alternatives I don't know about. There are definitely choices to avoid.
Even though I emphasize lightweight gear, simply buying the lightest option may not always be the best. I often choose items that add some weight to gain performance or durability. I sometimes pass up the lightest option because it is simply too expensive for me to justify.
Care must be exercised when buying gear at a mainstream retail store. Sales people may or may not be tuned into ultralight gear. Whatever the case, they are going to try to sell you what is in the store (except in certain very rare situations). Some of the best ultralight gear is available from cottage industries and simply is not available at retail stores. This situation is changing, but it really pays to do your own homework. Some retail stores have contracts with certain brands and don't carry others. No store can carry every brand.
Be aware when your own emotions are giving you foolish guidance. Maybe an idea has gotten into your head that a certain brand is the cool thing, and you really just gotta have it. Beware of misplaced brand loyalty.
Don't overlook the idea of making some of your own gear. Apart from saving money, you can customize your gear and gain a great deal of satisfaction. Sewing skills can be used to repair or even modify commercial gear.
Find quality sources of online information. I haven't reviewed all the options, and new and exciting things are coming out all of the time.
Tom's backpacking pages / [email protected]