Common belief is, if you can lift 12 good reps, increase the weight and go back to 8. When you can do 12 good reps with that weight, increase the weight and go back to 8...rinse and repeat...

There are certainly a lot of other methods / routines people like, such as heavy days / light days weekly routines, but, the first one has always seemed one of the easiest to get started with and do consistently. Maybe while doing that you can research other routines types.