Lock yourself in the store overnight and go home when people show up in the morning. You've got to protect the goods.

surely there's other people working with you that you can ask about how to lock up?

damn, not only does your store have a closing sale, and you're going to be unemployed after three months. But you're supposed to close by yourself :O

Perhaps they plan on training you before the day is over, or it's not as complicated as you're expecting it to be.

yikes that sucks

ask

Have a going out of business sale. Keep marking stuff down until it's all gone, then go home.

Fake an illness and go home. Then they will have NO ONE, MUAHAHAHAHHAAH!

Just ask. Ask where the keys are, if you need any codes. Or if there is anything special you need to do.

When I started my job I also closed on the second shift. But I had someone with me who told me what to do so I'll know next time how to do it by myself. Kinda weird if nobody told you what to actually you, the owner can't expect you to just know so that makes him very irresponsible and it will be his fault