How can I make my stomach flat?

What can I do to make my stomach flatter, I mean I'm already pretty thin but I feel like my stomach is a bit uneven. The top part of my stomach is pretty flat I guess, but in the middle right above my belly button theres like this sort of indent..? Not sure what youd call it, but its sort of like if you wrapped your stomach with string and the indent it would make in between the two sides is kinda of what it looks like. It's really not that noticeable to be honest it really does bug me because when I look at my side in the mirror it makes my stomach look like a top lump and then a bottom lump. I already do a lot of cardio since I'm a long distance runner, about 35 miles a week right now but I'll be kicking it up to close to 55-60 miles a week throughout the summer. I just kind of want my stomach to be more flat and not lumpy so itll be easier for abs to form. Sooo can I do? I'm a 14 year old male btw. Thanks, all answers appreciated.