Decide how long you will wait for tardy riders. Establish a fare. If the driving is shared equally, you don't need to exchange money. If one person drives, you can simply agree on a mutually acceptable fare, or you can figure what to charge riders by dividing the round trip mileage times cost per mile by the number of riders. Check your insurance coverage.
Some companies offer rate reductions for ridesharing.
Make sure your vehicle is in good repair and has enough gas. Ask about preferential parking.