BK LS430

Hello I am a newbe to this site and got my LS (02) about two months ago. I am looking for rims now and can not make up my mind between: 19's, 20;s, or 21's. Furthermore I do not know what tire size (width and height) I need for each of those fits, if I need spacers, and i have no idea in the different type of rims, for example 1 piece vs. 3 piece, or cast vs. forged rims. and what is a staggered rim? I would appreciate any help, but I would perferably like an LS owner with rims of my nature to tutor me and or advise me in this matter. Thank you very much for listening.