Error: Could not find or load main class net.minecraft.client.main.Main Java HotSpot(TM) Client VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

i'm on windows xp (ik old asf) I'm pretty stupid when it comes to this stuff. Can you please fix this error and tell me step by step how to fix it back to normal? I want to be able to play again.