If your Customers and Suppliers are all in your Accounting System already, all you need to do is set up the link between Tradify and your Accounting System, under Settings > Accounting System and then you go to Customers > Options > Import Customer from Accounting System. The Customers and Suppliers will come across automatically.
If your customers are not in the Accounting System, then it is a little more complicated.
Under the "Options" button on the Customers screen, you will notice you can download a template to use for importing data into Tradify manually. Simply transfer the data you have into that template and then point Tradify to the file through the Import Customers from File option.
If you want to import Job history, there isn't an easy to do this and is therefore not recommended, since the time and effort to do this would be excessive, especially when considering the limited benefit having the history in the system would offer. If you have access to your Job history in another system, then it is best to reference it there when needed.
If you want to import your current open Jobs, then you can import Job header information, using the "Options" button on the main Jobs screen, but the Job Costs and Invoicing details will need to be manually captured.