I just bought a Lumix S5II because I wanted to take better photos of my family but honestly im kind of overwhelmed with all the buttons and settings. My sister is getting married soon and she asked me to take some engagement photos for her at a park here in Seattle next month. I have the lens that came with the camera but the background isnt getting that blurry look that everyone likes you know? I think they call it bokeh. I read online that I need a prime lens for that but I dont really know what that means exactly other than it doesnt zoom.

I have about 800 dollars saved up for a new lens but looking at the options is making my head spin. I see stuff from Sigma and then some from Panasonic and the prices are all over the place. Is 85mm the one I want for this kind of thing? Someone told me that but then I saw a 50mm and it was way cheaper so now im just lost. I really want her photos to look professional and not just like I took them on my phone. Sorry if this is a really basic question but I just want to make sure I dont buy the wrong thing and waste my money because I cant really afford to buy more than one right now. Which L-mount prime lens should I get if I want those really pretty professional looking portraits with the soft backgrounds?