if you have a fully open aperture your bokeh will be entirely circular at lower apertures you can have sharper edges (based on the shape of your aperture)

basically it’s points of light reflecting the shape of the aperture when they are out of the images dof

so you want the f stop fairly low usually in the 1 range but you can also get the same effect in front of the subject image by increasing this

it’s all subjective to distance and the lens type you have. so just use those as basic rules

here this might be helpful
www.sony.ca/html/uf/microsite/Capturing_Depth_Minisite/