If you've got the time/money, I'd replace the pads and lines with new.
If like me, the money is an issue, you could start with trying to clean up the existing pads. Take them out and give a go over with sandpaper to remove the top surface. (Don't inhale the dust. It might not be harmful, but best be safe).
If that doesn't work, I'd replace the pads and if they don't sort it, you can always swap them back and keep the new set for spares when the originals do wear out.
After then go for new lines. Go for braided if you can afford it, it's worth the investment.
You might also have to look at clean/rebuild/replacing the master cylinder.