If the salt water chlorine generator (SWG) is showing green lights, suggest you not worry about salt levels now. Since salt doesn't evaporate, it should stay stable for a long time. If it is high from too much shocking, nothing bad will happen, as long as the SWG is showing green status lights. It checks for the salt to be within it's acceptable range, and turns off if it is not acceptable.
CYA is very low compared to the TFP recommendation for SWGs. however a lower level will make it easier to SLAM. But 18 is too low even for non SWG pools. The more knowledgeable people here will improve my recommendations, but I'd suggest you bump up the CYA to 40 (30-50) so you don't lose too much FC to the sun while you SLAM. Once you have finished your SLAM, you will want to move CYA up to the 70-80 TFP recommendation for SWG.
To step up to 40: do this by adding granular stabilizer (CYA). To figure out how much to go from your 18 to 40 use pool math (one of the 4 big buttons up top of this page. Pool math is your holy grail. It will tell you everything.
Here is CYA if you are unsure. Not sure it is a great price, but pretty close to what you need if it is a 20k gal pool.
Amazon.com : Pool Mate 1-2607B Stabilizer and Conditioner for Swimming Pools, 7-Pound : Patio, Lawn Garden
Update your signature with your pool size in gallons, location, equipment, etc (your welcome message suggests what to put and how to do it). But for now I'll assume you have a 20,000 gallon pool. If you don't know how many gallons, TFP has a pool volume calculator.
So assuming you are 20k gal, pool math told me you need139 oz to go from 18-40 CYA. Or 139/16=8.7 lbs.
hopefully you have your TF-100 over size kit on its way. If not, order that now; you will use a lot of the FC reagent, which is why I'm suggesting the oversized kit.
Once your kit arrives, start your SLAM. Follow the directions. Follow the directions. In the mean time, clean your filter. It is going to need to filter out all that trash.
Did i I mention to follow the SLAM directions? ;-). It will take days to even weeks depending on how bad it is.
Report back each day on your measurements and you will get lots of good guidance. If you follow the directions, you will never have to shock/SLAM your pool again. Pretty cool.
You are are going to need a bunch of bleach for the SLAM. Cheapest is get it at your local store. Just plain generic bleach, not any of the slow pour or oe fumed stuff.
So get that kit, update your signature line so we can help you better, and look forward to an amazingly clear pool.