How should I go about asking for a column?

Well, I'm not sure I'd phrase it that way to an editor. I'm a newspaper columnist, but I took a different tack. Since you don't say anything about your writing experience or background, I may assume some things that are not true; if so, just skip them.

You should have a number of clips including (ideally) ones from the newspaper or magazine you are targeting. You must also be familiar with the publication and its audience. (Knowing the editor personally and having the editor know you and your work is the best starting point you can have.)

You should be able to identify a need for the publication and its audience and be able to fill it. For example, if you are a gourmet cook and have published articles on cooking, restaurants, food trends, ingredients, etc. and they don't have a food writer or restaurant critic and the city you live in is large enough to warrant one, then that's an opportunity.

You should be able to prove you can meet deadllines without fail. Most importantly, you must tell the editor what you can do for her (or him) because--and this is the kicker--it is not what you want that matters, it is what the editor and publication needs.

I had (and had done) all that so when I sent my proposal packet off to the managing editor--with six clips, three sample columns, references and a carefully targeted cover letter--I knew it was right. His call to me, which occurred two days after I mailed the packet, confirmed that.