Retatrutide is a different animal compared to Semaglutide or Tirzepatide because of that third receptor: the glucagon receptor agonist. While GLP-1 and GIP handle insulin secretion and gastric emptying, the glucagon component in Reta is what actually ramps up thermogenesis and hepatic fat oxidation.
For someone starting out, the mistake is jumping to high doses too fast. The 'sweet spot' for weight loss without losing significant lean mass is usually found between 2mg and 4mg weekly. I'd suggest starting at 1mg or 2mg (split into two doses, like Monday/Thursday) to assess your heart rate response. Glucagon agonists can slightly elevate resting heart rate, so you want to adapt to that baseline first.
Also, keep an eye on your hydration and electrolytes. Reta is so effective at suppressing appetite that guys often forget to drink enough water, leading to that 'flat' look and lethargy. If you're over 20% BF, 2-4mg weekly combined with a high-protein diet and 40 mins of LISS cardio will melt fat faster than almost anything else on the market right now. Don't chase the 12mg 'max dose' used in clinical trials unless you're a pro-level athlete with perfect bloodwork.