Please read this section on renting an apartment, if you have not already done so.
収入証明書(源泉徴収票または納税証明書、給与明細書 - statement of income (either withholding tax statement = this is issued by your employer - or tax payment certificate, or statement of salary details)
Basically what the real estate agent is looking for is assurance that you have steady income to pay the rent regularly.
In any case, you will need so-called "guarantor" to say that person is going to be responsible for the rent payment in case you cannot pay, or to be jointly responsible for the rent. Normally among Japanese people, people ask their parent(s) to be the guarantor - this is a common practice in Japan.
I don't know where you are going to study at, but some universities/colleges can refer you to apartments/landlords that are happy to rent to students, and there are some estate agents who provide the guarantor service (against a fee). Ask your school what kind of help they can give you.
Once you find the apartment you want (that is available), you have the landlord's consent to rent to a foreign student, and if you come with the
school enrolment paper and ID, along with the initial payment required (in Osaka area, the amounts required as deposit and such can be quite an amount) it will be a matter of a few days to get everything done. So... if you can find in advance, through your school or through web search, an agency that can show you around to look at rooms immediately upon your arrival in Japan, you can maybe expect to stay in a hotel accommodation for maybe a week or so, because you might want to look around several places before you decide.
If you go and find yourself a 3DK apartment alone (which would be considered big for a single student), they will probably ask you if you are going to be alone or with someone - be sure to tell them up front that your brother/sister will be coming later, and how many will live altogether in the apartment. Landlords do not want surprises later :)