If you are installing from an ISO file you can choose to install the Evaluation Edition that is activated with 180-day expiration. Enough time for you to investigate and obtain the product key. When installing SQL Server on the Product Key page of the setup program choose Evalution Edition on the "Specify a free edition" field.
To investigate where is the product key try the following:
- If you purchased SQL Server directly from Microsoft or an authorized retailer, the product key is usually provided in the purchase confirmation email or on the retail packaging. Sometimes is also pre-pidded on the ISO media.
- For organizations with volume licensing, the product key can be found in the Volume Licensing Service Center (VLSC).
- You can also retrieve product key from existing installations as explained here.
Once you get the product key you can safely perform an edition upgrade to a licensed edition without any risk to lose any data.
Hope this helps.