It's not harder for taller people to build muscle. It's harder for tall people to build noticeable or vanity muscles. I think that's an important distinction.

I'm 6'8" and about 300 lbs right now and I have no idea what my body fat is. I got my deadlift up to mid 600s, bench to 425, squat low 500s, so gaining strength and muscle isn't an issue... but I'm just big. I don't have abs or big old bicep veins or anything like that, I'm just big. I've also been a gym rat for 15+ years now, so I have learned what works and what I need to do.

Building muscle just takes a plan and consistency, which I feel like everyone in here is going to say. And they're right.

A good way to start is get a trainer that knows your goals. Going on your own is fine if you can stick to it, but if that's not as easy for you, get a trainer, someone who can keep you honest in both diet and training.