As with any impressive physical feat, Super Saiyan and Ultra Instinct require a lot of training and dedication. Goku didn’t go SS overnight – he trained for his entire life up to that point (and after it’s worth noting). It helps to have a strong motivator to keep up the training (like the rivalry with Vegeta or an existential threat in Frieza). Just like Goku, I recommend starting small with some easy diet changes and incorporating exercise into your routine. Exercise classes can be useful because they will give you a social network to keep you coming back and teach you good technique.