How to pace yourself for 800m running?

tomorrow is sports day and i have to run the 800m for the girls. I don't go to a running club or anything so i don't really know "how" to do it. Last year i did the same race and came in second place. I was sprinting the first 400m and I was first until about 600/700 when the runner behind me overtook me. I know im a long distance runner rather than a sprinter (if that helps) but truthfully i don't expect to do as well as i'd done last year because of the lack of exercise i do. I've heard that the first 400 and the second 400 should be at a rough same pace but my dad (who's an experienced runner) says that i should go slow at first so i can sprint the last 100 m. thing is whenever i do the first bit slow im alsways still out of breath and i cant sprint or overtake anyone. Any tips?