To prevent any contamination of your spring, make a trench. Dig this across the slope above the spring box so no surface water enters the system. Pit latrines, dumps, defecating animals and other sources of contamination should not be located up hill above the source of the spring.
Many springs are found on slopes. If your spring has a healthy flow, you will need to build a spring box. Dig a hole just below the mouth of the spring. Be sure it is open at the bottom (or back, if on a steep hill). Make this spring box a square, around one yard by one yard by one yard. If you want gravel in the bottom of the spring box to help with filtration, add it now.
The walls of the box should be no larger than necessary. Cover them with a removable top to permit cleaning inside. Position a large pipe to drain off the water during construction of the box. Remove or seal this outlet when the project is finished. A smaller pipe is needed to handle any over-flow coming out of the spring.
You can use any water holding tank if it inserts easily into the spring hole. Remembe,r if it is filling from a run, a tank is fine, but if you are allowing the water to rise into the tank from below, do not have a bottom on the tank. Cement poured into wall forms, rock mortared spring boxes and plastic tanks, are all common approaches that work.