High Metabolism!

Corrin • Hey! Im a country girl of 19 years, coupled to a beautiful fellow female! 👭
Hi Girls! 
I'm a skinny, 5,11" 19 year old lady! 
I have a high metabolism as I can eat a load of food and not put on any weight whatsoever! If anything, I'm losing weight gradually over the last three years so far. I'd love to tone up and get fit without losing any weight.
Could I go to yoga or the gym? 
Does anyone have any advice for me? 
Inside knowledge : I'm studying Horse management at college which means that I'm on the go most days, lifting wheelbarrows, hay nets and riding the horses. I also loan a horse which means I'm awake at the crack of dawn, right the way through to the evening. 
Thanks :)