what do i tell the barber if i want to get my hair cut....but still want to grow it out?
Tell him exactly that--and it would not hurt to have some sort of picture to illustrate the maximum length you eventually want your hair to become.
First off, do you really want to go to a barber? or a stylist? You might have a more sympathetic ear if you go to a stylist.
Also, how long is your hair now? Is it already long enough for a pony tail? If so, then you might want to get the ends trimmed so they're all the same length, or just get the dead/split ends cut off. But if you're just starting to grow out your hair, then I wouldn't get it cut now -- wait until you can get it into a ponytail; otherwise, the time it takes to get there will be longer.
I didn't get my hair cut for over 3 years after I started growing it long. I finally got up the nerve to go in -- I had heard all kinds of stories about stylists cutting off more than what you want cut. So when I sat down in the chair, I told her exactly what I wanted, I actually grabbed my tail and showed her where to cut, and when I was comfortable that she understand, she finally trimmed it. I just got a quater to half inch cut off, and it sure looks a lot better. It's still not all the same length (since I didn't want her to cut off too much), but it sure looks better without the split ends.
Good luck!