The steps you need to follow are these.
Come up or adopt a naming scheme for your products (ie define your product codes if they are not already defined) and associate a barcode with each such product code
Update your product catalogue in Megaventory with the barcode information. Essentially this means that for each product code you need to fill in the EAN parameter with the barcode number you will be using for that product code. You can quickly do this by importing a spreadsheet in Megaventory (in Gear Icon > Products > Import).
Print the labels with the barcodes (and any additional information you need such as product code, price, description etc). You can configure your barcode printer to print in the 2x1 label roll you have as necessary - the instructions which came with the barcode printer should help you with that.
Stick the labels onto the products.
From that point on you can scan a product with a barcode scanner and pass that information to Megaventory. The hardware will recognize the barcode number and pass it to Megaventory which will associate it with the right product e.g. in order to paste it into a Sales Order or other such document.