Bmx mtb bike sizes helpppppp?

[Arthur]

Ok now im wondering since ive seen mtb prices and bmx im still cant deside what i want to do bcause right no i ride a regualr bmx im like 5'9 but my bike was made with like lil kid parts cuz it had like 5" bars and stuff but anyways i probly won't be aloud to spend more than 300ish but idk what i want and no one i no will let me ride there 24 cuz they saw my big crash and think ill break there bike and ive only seen 1 kid with a dirtjumping mtb and i only saw him once so idk what to do mabey go test bikes at a shop???o and does any1 no and i would like sum cheaper awnsers and i basicly am a beigener cuz ill i can do is dirtjump footjamtail whip barsipin and jhop_bunnyhop desent so idk what would be best but thats all it will get used 4 bsides to get to school