Dokroma, from your questions, I can tell that you are curious and interested in electronics. However, the problem is that you obviously have not studied and are not familiar with Ohm's Law or have electrical/electronic training. Ohm's Law is the very first thing that you learn when you take a course in electrical/electronics and one of the first courses that are required when you start is electrical safety and CPR training (at least here in the US). Meters have ratings that show the allowed maximums that your meter can safely sustain, so be careful that your meter is rated to measure the AC voltages of your mains. I have seen meters explode when measuring voltages exceeding the meter's ability. This may seem a bit hyperbolic, but mains voltages and current capacities are nothing to take lightly. Over time and experience, you too can safely do this, but training is paramount.
With that said, other comments here are trying to point out the relationships in Ohm's Law that pertain specifically to your questions, which is fine, however, I recommend that you take some structured courses in this field from a qualified school before engaging in the field too much. There are many people that decide to take this up as a hobby and do the basics, watch videos, look on the internet, etc. to learn as much as they can, but, they will tell you that it is not easy or quick. There are many learning videos on YT such as Khan academy and many more, just do a search. Without the structured training background, it will be very difficult to do the things that you are asking about. You have to realize that most of the experts on this forum not only have a 4 year degree (or greater) in this field, but also have many years experience. My only point is that if you are interested in the field, be prepared to do the work, because there is much work in learning electronics.
If you are only casually interested as a hobby, you will still need many hours of study to get started, but, DO NOT forget the safety training, for your own sake (and for your family or those around you). "The Art of Electronics" is a very popular book for the purpose of learning, but you should start with something more basic, like Forrest Mims "Getting Started in Electronics".
Electronics is a fun hobby, and can be a lucrative career, but it requires a great deal of study. If you don't put in the work to learn it, the return will be equal.
Hope this helps...