to give proper assistance it may help if you share what theme your using
i know with most themes i've used you can fix this kind of stuff in the lil sidebar on the theme customization window (the red box in the screencap)
if it's a page theme then you may have to reference (if the code maker has it) the guide for making changes to the page theme like columns and such
if it's your own custom code then i'm not sure as i've never built one from scratch myself (i havent done any kind of web coding in years and my knowledge is outdated so i wont be any help there i just kinda wing it )
circling back to just overall theme customization with the appearance options there's usually an option for sidebar and post width but again different themes means different options for customization. like i know tumblr defaults can be kind of a pain to fiddle with (at least personally) but other folks custom themes have functioned a bit better for me
i will say that theme options got kinda rough after tumblr made some kind of swap for the allowance of i think java coding so that gets real screwy sometimes but other than it's usually okay