At the bare minimum, you need some disc cones (20 or 30 is ideal) and some pinnies. Players can bring their own soccer ball. However, most coaches will accumulate additional equipment over time that can come in very handy. Optional equipment includes:
A coach's first aid kit
Pop up goals or even portable full size goals like GOLME
A dry erase clipboard with a soccer field stenciled on it. Handy for keeping papers together and drawing out things you may try to explain
A stopwatch - handy for keeping track of time during matches
A small ball pump
A colorful keeper jersey and gloves
Extra balls are always nice.  It helps keep the practice moving if you don't have to chase balls all the time
For older players - training aids like an agility ladder, hurdles, ball hoops, poles, etc.You'll notice one item that's not on the list - a whistle. Most experienced coaches won't use a whistle and instead teach the players to listen to what they say.