L.,
Last year I wrote an article on discipline for very young children (10+ months and older). I'm happy to send the whole thing via email if anyone requests it; just send me a message.
I think it is first very important to note that young children do not truly "understand" the word "no". What they do understand is that we use this relatively meaningless word with disapproval and anger, and often repeat it. But this is extremely confusing for children, mainly because we forget that "no" is not a positive instruction or direction, but just a word. When we say it in a big, loud voice, it can even feel like a game for children, almost as if we were saying "BOO!". This is why parents are regularly frustrated when their child won't listen to "no".
Here are some things that you can do:
Give a clear, one or two word positive direction. By this, I mean phrases like "Drop it", "Stop", "Sit down", "Let go", etc. This will help acquaint your boy with these words, but don't expect him to learn them overnight. Children need to hear words hundreds of times before they sink in.
The next step is to gently physically assist your son. If you have told your son "Get down", and he hasn't gotten down, go over and help him down, repeating the phrase a few times. (bear in mind, too, that in regard to climbing, some children actually can't get down and need our help). This is useful in reinforcing many directions.
If your child is holding something sharp or dangerous, do not pull it from their hand, but instead pinch the skin on the back of their hand just enough so they drop the item. This is important, esp if they are holding something sharp or dangerous, so that they can't tighten their grip.
Young children have very little self regulation or impulse control. I know a lot of parents feel that children should "just learn" what they may or may not touch/play with. However, my experience has been that parenting is phenomenally easier when we create safe places for our children to play by childproofing, putting "no no's" out of reach, and keeping children close by. Using gates and doors to their best advantage, adjusting furniture, etc have really made my life much less stressful, and the children enjoy themselves more because they are not constantly being corrected.
It is scientifically proven that very young children do not have the brain development to multitask. This is why you cannot get your son's attention. This isn't about him "not listening" as much as it is that he is thoroughly absorbed in whatever he's doing. Keeping that in mind helps us lower our expectations. 12 months old is still a baby, mentally. :) Try to join him where he is. "I see you like to hold the snowglobe--it's so pretty. Here, you help mama put it on the shelf" is a gentle but direct way of getting his attention by showing that you see what he's engaged in, and then correcting the situation to your satifaction. If he's sad, then you can try some empathy and redirection:"I see you are so sad. You really want to hold it. Let's look over here...oh, here's your ball. I like to roll the ball to you." Often, young children are pretty distractable if we acknowledge their feelings and help them become interested in something that is appropriate for them.
I strongly disagree with those who would suggest that pain is a great teacher, because while children do understand pain, they do not understand why they are being hurt. This use of pain as a teaching tool dreadfully undermines our children having healthy, loving relationships with us, and slowly destroys their trust in us. When we teach them that those who love them hurt them "for their own good", we set our children up to enter abusive relationships. We are the adults. No matter what our children's demeanor, there is nothing that justifies us not finding loving ways to parent.
I want to add one more thing: you mention you are having some hard times. Please, please take some time to take care of yourself. If you can find a loving friend who will come over and play with your son while you soak in the tub for an hour, do it. We moms really need time to refuel our own gas tanks, so to speak. It sounds like things are very stressful for you right now, and this is an awful time of year to be feeling that stress. Please reach out to your friends, family, church (if you go to one) or others in your community. I am having to learn how to ask for help, and it's hard, but I'm discovering that people are wanting to help. Pleae be loving and gentle to yourself, and thanks for asking your question. It's so great to want to change things for the better.
My very, very best to you.
H.