In most cases, the fiscal years last 12 months. If it is your case, you just have to define what is the last day of your fiscal year in the accounting settings. By default, it is set on the 31st December.
However, there might be some exceptions. For example, if it is the first fiscal year of your business, it could last more or less than 12 months. In this case, some additional configuration is required.
Go in Accounting --> configuration --> settings and activate the fiscal years.
You can then configure your fiscal years in accounting --> configuration --> fiscal years.
You only have to create fiscal years if they last more or less than 12 months.