Mainly just be properly equipped for the weather/terrain. Make sure to get a decent pair of hiking boots and break them in before your trip. Socks are important, too - need moisture wicking materials, so dont wear cotton socks. Don't bring too much food and water - i.e. Don't go overboard with it. 1 liter per person per hour is a good guideline. Be sure to actually drink it, too. Don't want to be carrying all that water weight the whole hike. Hiking poles can be really useful for difficult terrain, but they also just improve your efficiency by taking some of the weight off your legs. Bring rain gear like FrogToggs if it's likely to rain. You want to stay dry as much as you can.

If you're in the US, the national parks are really great. State parks are also a good resource. If you can make a trip out west to Utah/Colorado, the parks there are great (maybe wait until Summer's over to go to Utah, though). The northern parks are great too - Wyoming and Montana are really nice.