Pants versus Shorts
I always wear long pants, and my friends who only wear shorts think I'm crazy, and vice versa. Consider your priorities. If you are sensitive to poison oak, prone to off-trail bushwhacking, or concerned about sun protection, long-sleeved pants and shirts are key. If you stay on wide fire roads and slather on sunblock, you may be able to get away with short pants and sleeves. Either way, don't forget a hat, and remember that tube of sunblock.
Cotton is sturdy, but uncomfortable when it gets wet. I like the feel of cotton, but what I can't stand is the way it stretches when I've been sweating heavily or gotten caught in the rain. I don't like my shirt hanging down to my knees. Synthetic moisture-wicking fibers are an improvement over cotton, but expensive.
You may try hiking in whatever you own to start out with, then add a few garments at a time, giving the new stuff a good tryout before investing in a whole new wardrobe. Wear whatever feels comfortable to you. I like pants and shirts that are light, fairly tight, and synthetic. I can't stand wearing jeans, as they wick poorly.
Some of these dayhikes require nothing more than sturdy running/cross training type shoes. Just make sure the shoes aren't too skimpy in padding. You'll know if your feet hurt at the end of a hike that you need more cushion. My current shoes (for short hikes) are Salomon x-hiking mids. I love their lightness. For longer hikes, I'm a fan of Salomon's more sturdy boots. Heavy, substantial boots are fine, and preferred for long hikes along rocky trails. Take care to break in new shoes by wearing them as you go about your daily tasks for a few days, before you wear them hiking. And take some moleskin on your first hike, just in case!
Don't skimp on socks! Good socks will act as a buffer between your feet and shoes. You will experience less foot fatigue and fewer blisters with adequate socks. I bought 5 different kinds of socks and then tested them, and found my dream socks, made by Smart Wool. I buy the thickest, plumpest kind they offer. They are expensive, but last a long time.