There are numerous National Parks and Forests, Wildlife Preserves, State and Regional Parks all over the world. People will often travel from foreign countries to visit some of the more famous parks: Banff National Park in Canada; Yellowstone, Yosemite, and Rocky Mountain National Parks in the US; Lake Victoria in Africa. Occasionally the trails in these parks need some repairs and maintenance: picking up loose trash, moving trees off the trail, restoring rock retaining walls, etc.
You can help today! Visit your local national park websites and see if they have a volunteer day. Many offer multiple days in a season, and even some overnight projects!
When you help restore a trail, you will be helping to keep the local trails clean for many to enjoy. You will feel a special connection to the specific trails and park you helped restore. Also, often the entrance fee to the park is waved for those who are helping rebuild trails.
Get a group of your friends together to volunteer on the same day as you. Bring some extra snacks and water to share with the other workers. Be sure to thank the park rangers or official staff who helped lead and organize the volunteer experience. This may be a one-day event for you, but for the rangers and staff it is their job!
You don't have to sign up to "officially" help with the trails; whenever you go hiking simply be aware of extra trash and pick it up! Or move large sticks and rocks off the trail that shouldn't be there. If the signs say to stay on the trail only, then stay on the trail, don't wander off it into the nature or wildlife preserves.