Get cycling shorts that are comfortable and not too tight. Tight shorts press on the skin and the friction causes rashes. If you can get your hands on shorts with padding, than it is going to be a more comfortable ride. Get tights that are the right size; not tight and not loose. Do not wear underwear with cycling shorts as it can also promote rashes
Make sure that your shorts are clean because unhygienic conditions encourage rash conditions and bacteria can also develop. Change your shorts after every ride session. Wear loose and breathable clothes when you are finished with riding. Unhygienic conditions can cause a rash even if you do not ride a bike. Take a good bath before riding a bicycle. Clean your butt area thoroughly. Do not give the medium for germs to grow on this area.
If you feel a rash or a burn developing in or around your butt area, take a break from riding for a while till the skin soothes down. Too much riding and the friction can take a toll on your butt and you might end with a rash so uncomfortable that it does not let you ride for a long time. It is better to avoid that and take periodic breaks. If you feel friction while riding the bike, stand on the pedal and release your butt from the seat. This will provide temporary relief from the friction.
It is important to adjust your seat at an angle that is comfortable. Do several trial and error sessions with your bike seat to determine the optimal setting in which you are comfortable.