Telling it like it is

What do you think is the best way to tell your friends that you have depression. I'm struggling very much with this right now. 

Allow me to answer your question with a question - Why do you want to tell your friends? I ask this not to put you on the spot or make you doubt your decision, but because the why will have an important impact on the how.

Likely your friends, when you tell them, will be confused and uncertain, and they will take their cues on how to react from you. This gives you an enormous opportunity to frame your illness in whatever way you desire.

If you don't want it to be a big deal, and you want your friends to keep treating you the same way, just mention it casually, like "Oh man, I just got back from the doctor and apparently I have depression. That really sucks."

If you're looking for support and comfort, then you can take your friends aside (either one by one or separately), sit them down on a couch and say "Look, I have something to tell you. I have depression, and I'm going to need some help getting through this."

Whatever you do, you shouldn't beat yourself up if things don't go exactly right. For all the power there is in framing your own experiences, you can't actually control how people are going to react. Maybe you'll try and play it casual, and one of your friends will over-react and start wailing and gnashing their teeth. Or maybe you'll be super serious and one of your friends will get uncomfortable and try and diffuse the situation with an inappropriate joke.

If they do those things, it doesn't mean that you did anything wrong. Depression is nothing to be ashamed of, and if you don't want to keep it private, you have every right to talk about it as much as you like. (Or as little. You are not obligated to tell anyone if you don't want to. It is entirely reasonable to keep it as something between you and your doctor, if that's your preference.)

I guess the thing to remember is that presumably the reason these people are your friends is because you like and trust them, and thus, even if they don't react the way you want them to, the probable worst case scenario is that they react in ways that are consistent with the reasons you like and trust them in the first place. And if that is the case, then there is no wrong way to tell them except the way that feels wrong for you.

Finally, with all that being said, my main advice is that you not rely on the advice of some random internet weirdo, and, if this remains a problem for any length of time, to discuss this issue with your doctor or therapist. They have experience in this sort of thing, and can point you in the direction of further educational and supportive resources.