Take an existing arrangement to church to see how it fits with the schematics. Use elongated flowers like gladiolus, then round flowers like carnations or lilies. Use small filler flower minimally. A bow from wide ribbon can be part of the arrangement. Use loops of bows in the arrangement itself.
Have a clear idea of what you're creating then buy the necessary supplies, but be sure to buy extras if this is your first time. Use plenty of oasis sponges and don't skimp on the water. Lastly use the right size container and don't be afraid to cut the stems on your flowers down bit by bit.