Let's break this one down a little bit
First things first. Is there any reason you can't buy at the store in order to go and speak to him? I'm sure you could find SOMETHING in the entire store that you could buy, even if it's the cheapest thing there!
It's obvious that you're a bit nervous about speaking to him, and that's fine. What that tells you is that you care whether he likes you or not. So how do you talk to him?
My advice would be to be yourself. Yes, that old chestnut. If you're pre-planning your approach too much, you may forget what you wanted to say and then you're stuck looking a bit lost. If you act naturally, and let the conversation flow, you're much more likely to leave a good impression.
What should you say?
My recommendation would be to simply tell him that you think he's attractive. Ask him a little about himself, and try to figure out whether he's open to meeting up with you sometime.
Don't simply approach him to give him your number. Allow him to see you for who you are. If things go well, then by all means ask if he'd like your phone number. Alternatively, you could ask for his.
The final verdict
Remember the following:
- You CAN buy something from the store - just choose anything.
- Don't try to pre-plan too much - you may forget what you wanted to say and start mumbling.
- Be yourself. Be polite, ask him about himself, let him know that you're interested.
- Ask if he'd be interested in getting to know you when he's not working.
Also, remember that he's at work. He might be a little surprised that you're approaching him in this way. Play it cool, and have fun with it. If he's not interested, what's the worst that could happen? You don't shop in that store on a regular basis anyway! Good luck!
Oh yeah, here's a video to watch beforehand that might help you out. Some of the advice varies to mine, but it's a worthwhile watch I think: