How to get my swimming pool alkalinity levels down?

I have bought a fast set 6000 litre (12ft) pool recently and experienced a problem with algae, my pool went completely green, so I then emptied it completely, put a gazebo up to block out the sun to an extent, I then cleaned it, Filled it up again and got advised to put a small cup of chlorine in the pool every night before going to bed, that this will keep the algae away and stop my green problems, Ive also got testers for testing the levels of the chlorine, PH, and alkalinity. Ive tested this over the past 3 days after refilling my pool again and adding this cup full of chlorine every night. The chlorine levels are now fine, on reading the test and comparing it to the leaflet it says the chlorine is 'ok', although now I have and even more difficult problem, the reading for the alkalinity is at its highest and the PH is also quite high too. As you can imagine I am going out of my mind as Ive only been in this pool once in 2 months. I dont know anything about all these chemicals that are needed so I have no idea how to get this balance right, everything I seem to do doesnt work. It is my daughters birthday tomorrow and im struggling to find a way to get this sorted for tomorrow so she can go in it for her birthday party. Does anyone have any info that can help me get this sorted ASAP. The alkalinity levels need lowering so what can I use that I can pick up from a local store at 7pm, I know if its acidic I can use bicarbonate soda. And whats with these PH levels, what do they do? I am very confused! I will be very gratefulfor your input as Im pulling my hair out here lol

Also would this water be too harmful for my daughter and her friends to get in tomorrow?