how many ways to retrieve data from another table in oracle

ok SA, my question is short but i wanted to mean here that i am working with one table and needed to access the data/column value from another table.
thanks

ok, Actually i wanted to develop a Payroll Management System
but first i wanna calculate the salary of the employee every month
here is my schema : https://www.dropbox.com/s/krcwkb51rboqyx2/schema.jpg?dl=0

the above mention dropbox link shows the schema link

i am not good in database as u see that my schema is not a good normalize one.

first i would like to calculate the following table

Pay_Allowance
Deduction
salary pay

the Pay_Allowance table gives the total pay category of the employee
the Deduction table gives the category of salary deduction
the salary pay table describe the net salary of the month

so for the above mention table i would like to develop a good normalize schema. thats why i wanted to know the good way to make relationship table between the three table. so that, suppose i am working with Salary_pay table but needed to calculate the Gross Pay from the Pay_allowance table and the Total Deduction from the Deduction table in that case how i can merge the two table

what is the best way to work with in such case ?

thanks,
SA