Keeping tadpoles is easy. You need a container to keep them in. I like to see them so a clear-sided container is a must for me. Make sure it is clean and well rinsed. Keep the container in an area away from direct sun so it doesn't get too hot. I like glass because it is durable, but I have used plastic bug houses like this for short visits. They are quite inexpensive and have nice handles for transport.
Just about any pond in southern New England, where I am, has frog eggs in early April and tadpoles in May and June. Check along the edges along branches or plants growing out of the water. Use a small bucket or a pan taped to a pole to reach farther into the water.
Gently scoop out some pond muck and check for tadpoles. They generally move around quite a bit after you’ve scooped the water. Be sure to take algae and plant material from the same pond to feed the tadpoles.
No matter what you collect, put them back in their home when they have legs and still have a tail. They need to re-acclimate to their home before leaving the pond.