I'm putting this one first because this is the one thing that new hikers don't think about, and it's possibly the most important one of all. If you are hiking in the desert you will need a LOT of water. If you are hiking at altitude, you will need a LOT of water. If you are hiking in the lush, shady forest, you will need SOME water. But you will always need it, unless your hike is 10 minutes long and ends at a water fountain.The number of times I've met new hikers on the trail desperate for water because they only brought a can of Sprite to split between them is way higher than you might ever believe. Water is the one thing you will need the most when you hike. You can get by without food for a while, but not water. Bring it. Bring at least 1 liter per person for every hour you plan to hike, and if you can't estimate how long it will be... over-estimate. Much better to have too much than to run out.
No jeans. For the love of god, don't hike in jeans. You will be so unhappy. A simple rule of thumb for what to wear on the trail is "no cotton". If you have to hike in your favorite cotton tee, so be it... but if you sweat or it rains at all, that cotton will get wet and stay wet. It will smell, it will sag and it will rub if you are wearing a backpack. Any dri-fit shirts, shorts or pants will do. Just feel the fabric between your fingers -- even a blend is better than 100% cotton.
Depending on where you are hiking, you might need long pants to avoid being scratched by thorny plants or poison ivy/oak. Always good to know the weather and the climate, but if you aren't sure, you can always layer. A lightweight dri-fit tee under a lightweight jacket you can remove if it warms up. Convertible pants can be pricey, but its pretty nice to be able to have long pants that zip off into shorts.
Depending on the length of the hike and if there will be a lot of up and down... you might need an ultra thin pair of socks under your regular (non cotton) socks. That can really help prevent blisters.Do not wear brand new shoes you have never worn (especially boots with ankle support). I am sure that you think you are tough enough to withstand the intense blisters you will get, but it will ruin your hike when every step feels like jagged shards of glass are coating the insides of your shoes. Lightweight, water-resistant, breathable shoes are great for hiking... and if you plan on elevation hiking, ankle support will be necessary to avoid twisting your ankle on uneven footing.
This one is surprisingly important, not just for you, but for the folks you bring with you. It is ideal that you match the challenge of the hike to the personality of the person you hike with. If you find a cool-looking 6 mile, 2,000 ft trail on Trails.com and then spend 2 hours talking your overweight, smoking friend into going with you, be certain that your friend is a gamer. As in, has the mental fortitude to a) step outside their comfort zone and b) be able to deal with a challenge without freaking out. Freaking out is the same as panic and it's very dangerous for everyone.
I need to tell you guys, from experience, taking someone out of their comfort zone and asking them to do something they may not be physically capable of is asking for a lot of misery on all sides. And it can potentially be very scary. Absolutely, GO HIKING with this friend, but make sure you pick a trail that you are sure you are both going to be able to handle.Another thing about companions... have at least one. Hiking solo is dangerous even for experienced hikers. If you are going to go solo, make sure multiple people know where and when you are going. Show them maps of your intended route and a time when you think you should be back. If something happens to you when you are alone on the trail, the more details your friends and family have to locate you, the better off everyone will be.
There is no excuse anymore to go into any trail situation blind. The internet has given us so much information about hiking trails that you don't even need to visit a hiking-specific site like everytrail.com or trails.com. Just google the name of the trail you are thinking about, or the area you would like to explore. There will be trail reports. READ THEM. Look at the maps, read about how to find the trailhead, look at google maps in satellite view to orient yourself on landmarks.Oftentimes the trail report will give you all the hazards to watch for as well as the difficulty level of the trail. This is SO important to having a safe and fun time when you hike. Losing the trail or spending a lot of time arguing about which way to go is a buzzkill. Hiking mad or uncertain sucks. For everyone.