First clean it up and see where you stand. It may not need to be resealed. Then give it the leak test.
If you do want to reseal the tank, it's not difficult. clean off all the inside joints using a razor blade scraper, like you would use for paint. Then wipe the inside edges down with isopropyl alcohol to remove any film, grease or other material. The surface must be completely clean and dry. Get a large tube of aquarium grade silicone. You will also need a caulking gun. Then run a bead of silicon around all the inside edges. Have a large supply of paper towels ready to wipe up any mistakes. This is almost like caulking around a bathtub. Let it dry for a few days, and then do another leak test.