Near black outs?

So, this used to happen to me...maybe about a year ago and now it is back again. I stand up and it starts to get dark, then darker, then lighter again. I sometimes have to sit down before it gets lighter. (I can only think of one time though) It happens usually when I rise, and sometimes when I may be outside. I figured its probably because I'm not drinking enough water so I started doing that, its not as bad I guess. My brother had the same problem except he actually blacked out all of a sudden and it was because he didn't get enough electrolytes..so I plan on drinking more Gatorade, except I don't want so much sugar. Any other methods? It also may have to do with the heat, its getting really humid and soon hot and humid over here. So, I also want to drink a lot of liquid for that. My main worry is that I play football and the pads and the heat may cause me to black out during practice, which would look wimpy and could be dangerous since it will get so hot here by august. (cont.)