Feeling Lost at Work...

I started my first job last week, and it's been really tough... Everyone seems to know what they're doing, but I feel so lost... My boss has given me some tasks, but every time I think I have a handle on something, another thing pops up that I've never done before. It's like everyone here is just pretending to know everything all the time... Is it normal for new jobs to be this overwhelming? Should I ask my coworkers more questions or try to figure things out on my own?...