The maximum L/D values for existing aircraft span a wide range, from 7.5 for the Zodiac Ch701 with floats to 21.5 for the Berkut 360. Source: Light Sport and General Aviation Airplane Comparison and Harloff Performance Factor. This comparison comprises 93 types, and that is normally where a drag calculation starts: You find a similar type and use that as a baseline.
For the wing drag you calculate the lift distribution for several angles of attack (try XFLR5 for this) and get the induced drag for the weights and speeds that belong to those angles. Now you need to know your airfoils and add their drag. Next, add increments for the interference with the fuselage, for fairings, gaps and probes using data from this book. Yes, it is more than half a century old but physics hasn't changed since then.
Drag is normally not the biggest in-plane load on a wing but lift at high angle of attack. If you need to calculate loads for a structural design, make sure you don't omit that particular load case. Don't be surprised: That lift has a strong forward-facing component.
If you are new to this, try to calculate several of the aircraft for which data exists. Only by doing this you can be sure that what you do gives good results. Drag calculation needs some experience and is, when done well, a lot of work.