This kind of matching isn't doable in the Marketo UI-based import, if that's what you're looking to do.
Marketo's standard dedupe key -- not unique identifier, since you're new I'll start you off on the right foot -- is the Email Address. (Email Address is not in any way guaranteed unique within the database, but Marketo will use it to find the best-fit target lead from the set of zero or more matching leads.)
While other guaranteed-unique (such as Lead ID) or probably-unique (such as other custom String fields) keys can be used to find matches during an API-based import, it sounds like you don't have anything close to a single reasonable key -- except perhaps the Phone Number if you can make a handshake-guarantee that it's unique across the db.
Another angle of attack is to run all your leads through a webhook (that has access to the sheet, say if it's uploaded to your Marketo assets) and have it do a lookup based on some criteria. Then pull the tagged leads out of Marketo afterward. Fun!
Anyway, how, in human terms, do you intend to match leads in the 'sheet to leads in the db? It seems like you need to start there.
Obviously you can export from Marketo to another sheet and then do whatever nightmare VLOOKUP/HLOOKUP you want...
Thank you for all this info. These next couple weeks are going to be tough but if everyone is as helpful as you, I should be able to get through it by coming here to ask questions!