Hi Petal, I am doing P90X resistance bands that you hang from the door and use for doing modified push ups and things like that, because that is what my husband is doing and it is also nicest for the injured shoulder I have been nursing a while. But I also do resistance just using my weight - I do plank pose like in yoga, modified girl pushups (because I can't even do a girl pushup) and wall pushups and swats where I hold the weights or lift them as I do swats. When we do weights, I start with something that is a little lighter than I think I need, and if when I do 10 repetitions slowly and at the end I am still not feeling the muscles get a nice workout, I go to the next weight up. If you don't have a heavier weight, you can also do the exercise as slowly as you can so that you are really working the muscle and not using any momentum to get the weight lifted. I'm sure LaMa knows way more than me about this though, and I'm sure there are a bunch of great youtube videos on this!
Don't worry, my arms are my weakest part and it only took a couple months to get them a lot nicer looking and also they really are getting stronger, which feels really good! Just be careful not to overdo at first!