I'm having a hard time knowing whether or not I have made certain vows and if I ought to keep them.
Someone explained to me that when Jesus says "Let your yes be yes and your no be no," his point isn't that vowing/swearing/making oaths are wrong (which is what some people have taken that passage to mean). His point is that as someone professing godliness, you ought to be trustworthy--if you say you're going to do something, then you don't have to add "I swear to God" because you'd do it whether or not you added that phrase. Basically, your word is as good as an oath.
I'm not married. When I was younger, I'd sometimes say things like "I'm never getting married," or "I'm going to die an old maid," etc. Since I've said those types of things, and since as a Christian my word is a binding as a vow, does that mean I can't ever marry or I'll be breaking a vow?
Again, today, someone asked me if I had any hours available during the day to watch her toddler. I said yes. She said she wasn't sure if she wanted to put him in a program, or have me watch him during the day (hiring me). I nodded and said "Okay." I don't think I want to do that job, but since I said "yes" and "Okay" does that mean I vowed to her that I would?
People do sin. "If we say that we have no sin, we deceive ourselves, and the truth is not in us" (I John 1:8). People do make statements that they later regret. The answer is to either keep your word, or admit that you sinned in making the statement. If you can repent of other sins, why can't a person repent of making a bad promise -- especially one that was only made to herself.
For example, for the friend that asked you to watch her toddler. Tell her that you thought about it more and decided that it isn't something you really want to do; however, you don't want to leave her in a difficult position either, so you are willing to watch the child for a few weeks to give her time to find someone else.
Now, none of this is to excuse telling someone something with the attitude that I can always change my mind later. As Christians we strive hard to avoid sin. "What shall we say then? Shall we continue in sin that grace may abound? Certainly not! How shall we who died to sin live any longer in it?" (Romans 6:1-2). A better answer to your friend would have been, "Let me think about it and get back to you in a few days," since you weren't absolutely certain you wanted to do this.