The whole board:
or you could mentally separate it into three columns or rows.
You can have a maximum of one number shown three times in any column or row. For example we already have the number eight three times but the number nine is only shown twice. so we need another nine.
In the upper right hand corner we have three possibilities where another number nine can go. So we look at the whole board to eliminate the possibilities in our mind.
In this case, it was easy to figure out the where the third nine should go. It will not be that easy with every number. It is a process of elimination with all numbers one through nine both in columns and rows.
Since only one number of each can be in each square, you can also eliminate boxes where a number exists. If most of a row is done, you can also fill in numbers.
Lastly if you have a mini-square almost filled in you can obviously choose the last number to be filled in.
There are other hints you can pick on your own, but just wanted to share a few starter hints. Good luck.