I just wanted to say I agree with everything that has been said, the problem with pool store testing is you never know how much care they have put into doing the test, some pool stores are better than others about this on average their reputations are not so good. Since you said you want to get a jump on things and this is a holiday weekend I guess we will have to work with what we have or can get. First take another water sample back to the pool store this time dilute it with 50/50 with tap water and see what the CYA level test out as, if it still reads 100, they your CYA is very high and a near complete drain and refill may be in order. Second you will need a good Chlorine test that can test Chlorine levels while shocking, you may be able to find one of these locally, it will be marked an FAS-DPD Chlorine test, most likely to be found will be a Taylor brand kit. Do not confuse this with the plan DPD Chlorine kit.
Once we know the CYA level we can get an idea of where to go, but at a minimum your looking at a partial drain and refill along with a lot of bleach/liquid chlorine.
Ike