The easiest way to create a number of products at the same time is to use the spreadsheet import.
But first, backup your existing product database! EXPORT all of your products - Products/Export (blue button at top). Export type is Product Excel, Export Criteria is All Products. Export, and save this on your hard drive as a backup (the "Backup Spreadsheet"). You never want to do a spreadsheet import without first backing up what's already in the database - just in case.
If you have not already done so, please watch the Basic Product Creation video before jumping into spreadsheets. Understanding the fields will assist with your spreadsheet preparation.
Once you have watched the Basic Product Setup video, the following 8-minute video will provide a good demonstration of the entire spreadsheet product creation process: