If you're just starting with working out and dialing in your diet, SARMs probably aren't the best option right now. They come with risks (hormonal suppression, liver strain, potential long-term effects), and at 315 lbs, you're likely to see significant progress just by focusing on nutrition and consistency in the gym. A solid calorie deficit, high protein intake, and strength training will yield far greater results for fat loss and muscle retention than any SARM at this stage.
If you're looking for something to help, I'd suggest focusing on natural approaches first, such as good sleep, hydration, and perhaps supplementing with creatine for a slight boost. And if you're still considering SARMs in the future, I recommend revisiting the idea after you've established a strong foundation of healthy habits.