I'm looking for a prime lens for my Nikon D850 but honestly I have no idea which one is actually the right one to get first. I just got this camera used from a family friend and it seems really fancy, maybe too fancy for me right now lol. I know a prime lens means it doesnt zoom but thats about it.

Sorry if this is a really basic question but I'm just so confused by all the options out there. I really want to take some nice pictures of my kids playing in the backyard and some portraits of my wife when we go on our trip to Maine next month, so I want that look where the person is sharp but the background is all soft and blurry. My budget is around $400 or maybe $500 if I stretch it.

Someone mentioned a 50mm lens is a good starter but then I saw people talking about 35mm or 85mm and I dont get the difference at all. Does a higher number mean it's better for portraits? I dont want to have to stand super far away just to get a shot. I'm just looking for something easy to use that will make my photos look professional instead of just like phone pictures. If anyone can explain which one I should buy for a beginner I would really appreciate it...