At 180 your a bit heavy to race a 250F competitively against 150 pound kids. Get a 450F or a 250 2 stroke and get some seat time at your favorite track...
First time doesn't matter. There's a beginner class in every cc range as well as age groups for youngsters or old guys. You should be in the beginer class for your cc size.
At 180 you will be at a disadvantage on the starts in the 250F/125 class. If your competitors are truly beginners it shouldn't make to much difference at this stage.
If you choose a 250F you will be on a 450F once you improve your skills.
so would i be better off on a 450 because of my weight? i know how to ride just never on a track. i use to race my friends on my crf 250. so i know how to get a good take off.