When you work out the contract with each client (and yes, you should have a contract in place!), you'll list out the terms of how often you plan to invoice and when you should be paid. For instance, you can agree to bill on the first of each month, or biweekly so you get paid every other Friday. Keep in mind whether your industry is one where you invoice upfront before any work has been completed. In this case you'll be detailing the progress of the project as opposed to doing a monthly invoice.
While you can make up your own invoice or use a free template, it's often easier to use an invoicing system that offers more in depth features. Some invoice systems, like Harvest, offer the option to create estimates for big projects, keep track of multiple clients and send reminder emails when payments are late. Another option is Freshbooks, which offers similar features but includes profit and loss sheets, as well as trackers to help you manage your time. Both of these services can be used for free or upgraded for a small fee, and include lots of other cool features, like scanning receipts for work completed in the field.