I won't really give you the query now, because this is really basic, but there is something that is called a "complex query" which is a query made by mixing 2 or more queries to find information quickly on a "big" database, so I will just give you the idea and with it you will be more than able to figure it out, or at least read a bit about SQL Queries.
If you want to find an "account name" you can use one toon's name of that account and look for it on the chars table, now that you have the toon's name, then you go with it, and look for the account, having the data that can be used on both of the tables you are going to use (chars and users_master) you can combine the information and bring up the account name.
This can be done not just with this kind of information but when you learn to use these kind of queries, you will be capable of finding any information you need having just one parameter easily.