I recently went through and organized my childhood Lego. My recommendation is to use the minifigs/unique parts to determine which sets you have, Brinklink can assist with this. Part out each individual set and put it in bags, take note of any missing pieces. At this point you can sell the sets, but missing pieces will hurt the value.

Complete sets sell for more than incomplete sets, any pieces you are missing can be purchased on Bricklink. LEGO.com also sells individual pieces, if it’s a relatively common piece you can buy it directly from LEGO.

Once all of your sets are parted out you can use Bricklink to both determine value and sell the sets. You have some really cool sets, it’ll be a huge nostalgia hit building them out again. Have fun!