Newbie here- how do I fix cracks in my fondant??

Did the fondant crack when you were rolling it, or when it was on the cake.....

it cracked after it was put on the cake-

The trick to getting crack free fondant cake is to knead the fondant properly before placing it on the cake. Underkneading usually causes cracks.

you can put a small amount of water and just pass your finger on it gently until the crac is erased

add shortening to the fondant before you start to roll it out. that will help keep it soft and helps with the cracking issues when it's going over the cake