It may be late when you worry about things like taking a test at school. Most likely your periods and cycles will become more regular as you grow older. There may be signs. There may not be. Sometimes there's a crampy feeling in the back, legs, or abdomen.
Some girls get pimples a few days before.
As you get older, you will become more familiar with signs that your period is coming. This will help you be prepared. Some girls have cramps with their periods. They usually get less and less crampy with time. Exercise may help prevent cramps.