Detrending data

[crmau]

Hi, I am working on a project in which I need to deseason and detrend the data to analyze it, being my variables sales and time. I can see after plotting that the data has a slightly positive trend with cyclical variations, so I need to use the additive model. My data is cut off so I start with September through December, then I have three full years and then again I have January through August. I'm not sure what type of model would fit the best but I was thinking about a linear regression model or can I start off with moving averages even though my first and last year aren't complete? Thanks in advance.