While it may sound boring, start with at least a skim of the manual. Have a look at the different triggers you can use to start your macro, the actions available, the tokens you can use in those actions, and get an idea of how all those things fit together.
Hopefully you've spotted the FinderSelection token, which you can use in an Open a File, Folder or Application action to open the Finder-selected zip in Bandizip. What happens then will depend on how Bandizip behaves -- if it auto-unzips the file to the same directory you don't need to do more for this step, otherwise you could drive Bandizip's interface using Select or Show a Menu Item actions. Then use Quit a Specific Application to Quit Bandizip. When you've finished you can use the Delete a File action, again with the FinderSelection token, to delete the old zip file.
Then you just need to decide how to trigger your action -- you could, for example, use an Hot Key trigger such as ⌃U.
You can use a similar process to write your zipping macro -- break it down into the steps you'd do manually then look for the actions to use and the tokens that provide "targets" for those actions.
Have a go and see how you get on. If you get stuck and can't find what you need in the manual then post your macro, as far as you've managed, and explain what your problem is.