It sounds like you are starting out with dayhiking. This is a good thing to do before you decide to spend the night on the trail. Most backpackers start this way. They do longer and longer hikes and then realize that if they take overnight gear, they can do even longer hikes.

For day hiking, just about any school bag will do. Look at the 10 essentials for what to pack, then find a trail close to home and HIKE! Just walk down it a couple hours, then turn around and walk back. If you enjoy it, then repeat. You will learn pretty quick if you like walking in the woods and whether you want to invest $$ into it.