You need these. They are a must. You could either get car seat belts that are 3 point harness. Or buy 4 or 5 point seat belts used for racing. E-bay is a great place to find cheap ones.  If you have money then get a 5 point seat belt. For starters, I got 3 point car seat belts since they where cheap, $10 each.

FIY: professional racing belts can be used for races only 2-3 years from manufacturing date, after which they expire. These belts can be found on ebay for a great price.