So, I started going to the gym about a month ago. Doing dumbell curls I started at 10lbs. I tried 50lbs at first.
Mistake!
Sized down until I got to 10lbs. I was looking for the first one I could do 20 reps. Then, as days went on, I kept moving up sizes. I'm currently at 25lbs. I keep wanting to move up to 30lbs, but my body gives way around the 12-15 rep range. So, not quite there yet. But the first 10 reps of 25lbs aren't even hard anymore. It only gets increasingly more difficult after the 10 mark, but more often than not the 15 mark. 15-20 are the hard ones.
But with 30lbs it's hard right from the start.
So, should I be doing the 30lbs reps, even if its less than 20 reps? Or do the 25lbs reps which I can easily do 3/4ths of before it becomes a struggle for the last few reps?
I would recommend focusing on a five or six lifts (don't skip the legs) and hyper-focus on good form. I like lifts that focus on large muscle groups. Things like deadlifts, bench press, squats, etc. Be sure that you get very good at form before you test yourself with weight. Do this for a year or more before you change things up. Focus on big muscle groups before you get good at the small muscles. Try to develop very good form.
On a separate part of the day (several hours apart), do 45 - 60 minutes of cardio exercise. This should be "easy" cardio. Like a fast walk, slow job, bicycling, etc. and the pace you should set is one where, when you talk, it is very obvious your are exercising, but you could still carry on a conversation.
The reason why you don't do that at the same time as your lifting is it'll otherwise cut into your strength gains. Keeping it separate by a few hours reduces that risk.
Also, don't forget that it is impossible to out-exercise a bad diet. Keep up your intake of lean protein and fiber. Avoid saturated fats and sugars. Healthy fats are unsaturated fats. Your body is a fire. The more fuel you put into it, the large it gets. The laws of thermodynamics support the idea that, if you want to lose weight, you need to eat fewer calories.