Question on tickets for live shows

[tsumekaze_]

Hello everybody!
Used to write a lot over here, but lately I haven't been writing much, though I still come and read a lot. Today I have a questions for the lucky ones who already know their way around tickets for live shows.
I managed to find a way to go and study in Japan for seven months, I will be there from April this year 'till October. I want to go to MANY live shows, but I do not have a clue where I should start looking for to buy a ticket.

Where do you buy tickets? where can you find cheap ones, or good seats?
Do you have any suggestion?

I don't belong to any fanclub. I would love to see ayu live, of course, also Koda, Namie, Ai Otsuka, Kana... these are the main ones.

Thank you very much, I'm sure there might already be some threads about this but I can't find anything with the search function!