Something is right up your alley if it's similar or the same as your job or hobby, which therefore makes you very knowledgeable about it. If you work with computers, for example, and a friend is having problems with his computer, you can say, "I can help you figure out what's wrong. That's right up my alley."
An alley is a tiny street usually between two buildings. Your alley is perhaps where you live and where you can find your interests and the things that you are good at. So when you say it's right up my alley, you're saying, "That's my expertise," or "I know how to do that very well."
A: Do you know anyone who can help me with my computer?
B: What's wrong with it?
A: It just suddenly quit working.
B: I can help you with that. That's right up my alley.
A: Oh, really? Awesome. I'll bring it in tomorrow.
B: Ok. I'll take a look at it.
A: Maybe you can help me with this Webpage that I'm building, too.
B: You should talk to Jack about that. He designs Webpages for a living, so that's right up his alley.
A: Really? Ok, I'll give him a call.
expertise - being an expert at something
Alright folks, think of something you are very good at in your job or as a hobby. Then you can say that's right up my alley if a friend needs more information about it.
Enjoy your day.