When you're just getting started with weight training, it's important to know the basic strength training principles. These are pretty straightforward and can be helpful in figuring out how to set up your workouts so that you're always progressing and avoiding weight loss plateaus.
Overload: The first thing you need to do to build lean muscle tissue is use more resistance than your muscles are used to. This is important because the more you do, the more your body is capable of doing, so you should increase your workload to avoid plateaus. In plain language, this means you should be lifting enough weight that you can only complete the desired number of reps. You should be able to finish your last rep with difficulty, but also with good form.
Progression: To avoid plateaus or adaptation, you need to increase your intensity regularly. You can do this by increasing the amount of weight lifted, changing your sets/reps, changing the exercises, and/or changing the type of resistance. You can make these changes on a weekly or monthly basis.
Specificity: This means you should train for your goal. If you want to increase your strength, your program should be designed around that goal (e.g., train with heavier weights closer to your 1 RM, or 1 rep max). To lose weight, you might want to focus on circuit training, since that may give you the most bang for your buck.
Rest and Recovery: Rest days are just as important as workout days. It is during these rest periods that your muscles grow and change, so make sure you're not working the same muscle groups two days in a row.