I sprayed WD-40 in ny keyhole to stop my key from getting stuck inside. Now my knob is so loose that I don't even need a key to open the front door. I just twist my knob 2 or 3 times and the door unlocks. How can I go about fixing this so other people can't do the same to my door and walk right in?
From the sounds of it I would suggest that it is time to replace the front door lock. Be aware that there is a huge range of quality in door locks and door knob units. The lowest quality ones are often what is found on mass built homes, condos and apartments because it saves the builder a lot of money when they are buying 100's at a time. The lower quality ones are more apt to give you trouble as well.
Since you are only looking to fix one door lock it would be worth it to look over the available units and select one that is better quality. That often comes with a bit higher price as well. It has also been my experience that higher quality units tend to have a much more solid feel to them and contain few or no plastic parts.
You very well may wind up repairing or replacing this lock as suggested in the other answer. It might have been shot before you did anything, with the pins broken off inside - you just never tested it before.
It's also possible the lubrication, along with making some parts move more smoothly, turned some solid dirt that wasn't really causing any trouble in there, into liquid gunk that worked into places it does cause trouble. If that's the case, you might double down and spray MORE lube in there, and / or compressed air from a canned air duster, may clear it out - longshot but at this point why not.
If this lock is keyed the same as your deadbolt, or other locks on your house, I'd just take the lockset / doorknob off, take it to a locksmith, and have it re-keyed to match the other locks. That is usually quick, easy, and inexpensive. The locksmith will replace the pins anyway, so if those are gummed up or broken they'll get fixed. It might very well be better than new if the locksmith uses better parts than the manufacturer used.