How Can I Avoid Getting a Misdemeanor Charge for Doing Drugs on Federal Property

My question involves criminal law for the state of: Georgia

Hi, i am 18 and i made a stupid mistake. i was smoking spice at a national park (federal property). i have never had any criminal charges of any kind. i am a very good student. i already got into the university of y choice. i have a green-card. now, the spice itself isn't illegal, but i had a bong and it had traces of marijuana, so i think they're gonna charge me for possession. the stupid part is that i called the cops on myself because i was so freaked out (i don't know if that matters while pleading my case). i have to go to federal court to defend myself. what should i do? i am a pretty bright kid with a lot of promise of a good future, and i don't want a misdemeanor on my record. please help me get rid of it.
if it isn't possible to get rid of it, could you tell me what consequences this will have on my future? will it affect my residency status in the US?
thank you.

Hey bright kid: Get a lawyer.

Retain a lawyer or see if you qualify for a federal defender. Federal charges are nothing to mess around with.

That last bit - you're a permanent resident? Here on a visa?

thank you, i will definitely get a lawyer. it's just very expensive.
i am a green-card holder, I've lived here for more than 10 years.
can you tell me if there is a way to completely erase this from my record? i haven't been able to sleep, i just need to know the best and worst case scenarios.

Worst case: At least $100,000 fine and a year in jail.
Best case: Dismissal.
Most likely: $1000 fine and probation.

If convicted it's NEVER going to erase from your record.
It shouldn't by itself screw your permanent residence.

thank you. i can only hope for best case, but i think i can live with the likely scenario.