It's not hard.
You need a remineralisation salt such as
Tropic Marin Re-Mineral Tropic (caution, they make a marine one too). Both Ebay and Amazon sell it, and probably some shops local to you will stock it.
Then experiment with a bucket of water and your GH tester. Add the salt a bit at a time till you get the GH you want. You will know the volume of water in the bucket and how much salts you used in the bucket, so multiply it up to the volume of your tank.
The advantage with these salts over crushed coral etc is that they contain all the minerals found in hard water not just calcium carbonate.
Once you have fish, when you do a weekly water change it would be easiest to refill the tank using the bucket and make up the water to the right concentration in each bucket before you add it. After a few water changes you will know exactly how much salts to use.
But make sure you always have a spare pack in the cupboard as you won't be able to do a water change with either plain tap water or spring water as that would change the mineral content of the tank too much.
If you decide to do this, start now before you have fish!