When you wonder about something, you want to find out some information about it. For example, if you say, "I wonder what the weather will be like tomorrow," it means you don't know what the weather forecast is for tomorrow, but you want to find out. No wonder is sort of the opposite because you don't need any more information. In fact, when you say no wonder, it means you already know the reason or the explanation for something. No wonder is similar to "that explains it" or "that explains why ..."
So if your friend looks tired, and you find out that he had been studying all night, you can then say, "No wonder you look so tired." Sometimes, no wonder is used negatively to place fault on someone, especially when you're angry. For example, if someone lost an important game, and you know it's because he doesn't like to practice, some people might tell him, "No wonder you lost. You never practice." This is, of course, not a pleasant thing to say.
A: I had no idea he works two hours away. No wonder he leaves his house so early.
B: Yes, he's the president of some company.
A: I wonder why they don't just move closer to his workplace.
B: Well, they like the neighborhood and the school district.
A: They're such a nice family. I never see them on the weekends, though.
B: They have a house in the country. They usually spend the weekends there.
A: Oh, no wonder I never see them.
Alright folks, find opportunities to use no wonder. Remember, it's important to practice as much as you can.