How can I improve the chances of my questions to be more visible for those who can answer me? I know the use of tags help a lot, but sometimes you can't figure it out which tag is better for each question. Can anyother person tag my questions? Is another way to relate questions beside tags?
I was a bit hesitant to post an answer here, mainly because I find the question a bit unclear and unfocused. (It seems to be about several completely unrelated things.) But after the OP's request in his comment I decide to try to expand the above comments to an answer. I will do my best to address several points asked in the question.
There are several older posts here on meta asking what to do to get more attention to a question. For example, How to grab users' attention on an old question?. Other posts linked there might be of interest too.
This question seems to be focused slightly on another aspect (emphasis mine):
How can I improve the chances of my questions to be more visible for those who can answer me?
I know the use of tags help a lot, ...
So the question is how to increase the chance that experts from a given area see the question. I guess all of the advice about getting more attention in general apply here, too. (Advice given in those questions seems to be mainly asking clear and well-formulated questions, using bounties, if you have solved some part of the question yourself or learned something new which can help, editing the question or posting your own partial answer.)
Tags. As you have said yourself, correct tagging can also help with this. Many users can have tags related to their area of interest among their favorite tags or they can even have email subscription to questions from that tag. So this improves your chances that they will see your question.
Chat. If you have a look at chat rooms associated with this site, you may notice that some topic and mathematical discipline have separate chat rooms. Posting your question also in the chat room associated with some specific area (assuming that such chat room exists) might also get you a few views from people interested in that area.
But you should definitely not overdo this - wait a bit before posting in chat. (If every question from main site is posted in chat, there is no difference between looking at questions mentioned in chat and questions on the main site.) I should also add that several of those chat rooms had periods of higher and lower activity. Currently most of them are not very active.
Time. There was even this older post on meta: Depending on the tag is there a possibility to know on which days and hours are there more answers? You can look at the findings given there or try the SEDE query in that post for tags of your interest.
While in theory it is possible that, for example, more experts in set-theory are in Europe and more experts in general-topology are in the USA and it might make slight different whether you post your question during daytime or night in USA (or Europe), personally I do not think that this really has big influence.
And I should add that my example with the two tags is entirely hypothetical - I simply wanted mention some two tags as examples. Although it is true that the user which has currently largest score in set theory resides in Europe and the user who has currently most reputation in general topology lives in the USA. But if you have a look at other users who are among top contributors in those tag and have mentioned in they profile their country, you can see that there are many of them from various parts of the world.
...but sometimes you can't figure it out which tag is better for each question.
Can any other person tag my questions?
Yes. Every user with at least 2k reputation points can edit your question, including changing tags. And user with lower reputation can make suggested edits, which means that the edit is not applied immediately.
So if you are not sure about correct tags, you can rely on help of other users. It might help if you explicitly mention in a comment that you were not sure which are correct tags for your question. (Another possibility would be asking in the tagging chatroom or in the main chatroom, but I think that mentioning this in a comment is more than sufficient.)
For some advice on how to find correct tags for your question, have a look also here: How am I supposed to use tags?
A closely related question is at https://math.stackexchange.com/help/no-one-answers. They propose the same approach Surb did.