a tank planted with questions

[dippy]

I have setup my 20g planted tank. I have a couple Java ferns and some java moss and a few others that I don't know the names of. I have checked some things and the ph is fluctuating from 6.0 to 6.5. The General Hardness is 300+ ppm and the Alkalinity is about 0 ppm. I don't know much about these things but the tester said these weren't especially good readings. The tester recommended Gh 50-150 and alk. 120-300. The ph I try to keep at 6.5 for the fish and the local aquarium store owner said it would be alright for the plants. So...how do I lower the Gh and raise the Alk?