Stick with the group classes if you like them. Otherwise   Meet with a trainer for a few times to learn form on deadlifts, squats; any compound movement. Take it slow and work up. Make sure you take some branch chain amino acids in your water to encourage that muscle to develop. Eat a protein meal after-typically you want 0.7-1g of protein per pound to gain a bit of mass. Specific examples of that you can easily find online (e.g. chicken breast is about 35g of protein but obv serving size matters).  Make sure you get enough sleep. Don’t be afraid to ask gymbros if you’re not sure how to do something. And if something actually truly hurts STOP what you’re doing-your form is likely off or you’re trying to lift too much weight. People are super susceptible to injuries when they first start lifting because they try to push their bodies too fast.  Lastly, take before and after shots-you’ll be pleased in a few weeks when you start seeing results!