Come to terms with ignoring the scale. Weight lifting isn’t going to make you lose pounds. Muscle weighs more than fat, and since the point of lifting strong is to gain muscle, chances are you’re actually going to go up a few pounds on the scale. However, it will make you look better: pounds of muscle will look thinner / slimmer / trimmer than pounds of fat, so it may look like you’re “losing weight” when you aren’t. Remember: the number on the scale is meaningless. Use measurements in inches (gained/lost), or use your overall health, or what you can lift: don’t use the scale as a success metric.
In general, clean up your diet. If you’re going to be lifting weights to build muscle, you have to fuel that muscle, and you need to fuel it right. You shouldn’t be in a huge calorie deficit – make sure you’re eating what your body needs every day. Calculate your BMR and your exercise expenditure and make sure you’re eating enough. And if you feed your body crap, you’ll get crap: focus on cleaning up that diet too. Lean proteins, vegetables and fruit, complex carbs: don’t lift weights and eat trash; don’t just eat more, eat better. If you don’t eat enough, or you don’t eat right, you’ll be wasting your time with a weightlifting regimen. You don’t have to count every calorie; you do have to pay a little more attention.
In specific, eat more protein. If you’re going to be lifting, you need to feed the muscle itself, specifically, by eating protein. If your muscles don’t have the protein they need, they won’t grow, and then what’s the point? Some guidelines say eat 1 g protein per lb body weight daily. Others target overall food macronutrient consumption: 40/40/20 protein/carbs/fat. The basic rule is, if you’re gonna lift, get your body more protein or – again – you’re wasting your time.
Lift heavy (if you can); Push hard. In general, lifting heavy weights for fewer reps is more efficient than lifting light weights for many reps. You’ll build more muscle faster with less time at the gym. Be aware of the possibility for injuries, and know your body’s limitations, and work with them. But in general, aim to do fewer reps at a higher weight. You don’t necessarily have to work to ‘failure’ every single time, but pushing through a set with heavy weights until you legitimately don’t feel like you can push any harder means you’ve actually done something. And lift heavier, push harder, the next time you’re in the gym.
Do compound lifts with free weights. The machines may look less intimidating, and doing weight machines is better than doing no weight-bearing work at all, but the machines are actually less efficient. They isolate muscles (rather than using multiple muscle groups and getting a full-body workout), and they stabilize you by having you sit or lie down (again, using less overall muscle, balance, and core strength). Learn to do compound lifts like squats, deadlifts, lunges, presses. Learn to do them with free weights: barbells, dumbbells, a squat rack.
Remember your rest days. When weightlifting, when you push hard through a heavy set, you’re actually microtearing your muscles; it’s in healing these microtears that you’ll actually grow muscle and build strength. This is what the protein’s for, but the muscles can’t build themselves back up if you don’t give them a rest day. Make sure you aren’t overworking. That healing period of rest in between workouts is when your body is actually growing stronger!
Be as consistent as you can. Lifting weights once a week every once in a while will still be good for you, but you may not see any significant gains. Try to stick to a schedule of 2-3 times per week, and try to stick to that for a period of 6-8 weeks at minimum. This kind of change can be slow, and it depends on what kind of fitness you’re coming from. Don’t give up if you don’t see any changes within a week, and don’t quit if you have a bad week or miss one workout. At the same time, don’t skip your workouts and wonder why nothing’s changing! Stick with it.