Be yourself around the boy you're trying to impress. This may sound like unhelpful advice, but the worst thing you can ever do is pretend to be something you're not. People will see right through you. So lighten up, enjoy yourself and have fun. Remember to laugh.
Find out what his interests are. This doesn't mean you need to pretend to be interested in the same things (unless you actually are), but it will give you a way to start conversations with him. Maybe you can even poke fun at him about something you learn; playful teasing is flirty and fun.
Dress appropriately. Often girls think the best way to lure a boy is by showing way too much skin, but dressing scantily could backfire. You never want to be (or seem) desperate for attention, and wearing inappropriate, or too little, clothing is a sure fire way to get the wrong kind of looks. Wear cute, age-appropriate clothing that leaves something to the imagination.
Lend an ear to him. Everyone needs a good listener in his life. If you like this boy, you don't want to nervously talk his ear off. Listen to him and be a good friend. Being relaxed and fun is intriguing, and if you're confident that you're fun to be around, the boy you want to impress will feel that. It won't take long for him to go from "boy to impress" to "definitely impressed."