how to get started with heavy lifting

I have started back using weights and adter seeing the results from lifting heavy i want in.
I have two dumbells but they only have 0.5kg weights on them and i generally do various exercises doing three sets and 12 reps.
I do this three times a week.

Now my question is what weight should i use? Or do i just keep upping it as i go along?
Also how many sets and reps should i be doing of each exercise?

I only workout from home so dont have access to machines, only free weights.
Thanks

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