Well, first thing to do would be to make a backup of the files. Its always a good idea to make a lot of backups up working files.
You would get CSS Form Builder with the extension library membership. You would want to install that and go to Insert->WebAssist->CSS Form Builder->Client Validation. then you would choose the checkout form and then you would add the validations you want for each of the forms. It might be a good idea to go through the Form Builder documentation in the support pages before applying any validations: