Practice and Review
The key to Sudoku is to always remember that there can only be one of each number from 1 through 9 within each row, column, and subgrid (referred to as "region"). If you keep that in mind you can use the numbers that are already in place (called givens or seeds) to deduce the numbers that go in the empty spaces. In most Sudoku puzzles you can usually start off by looking at the given numbers and observing which number has the most instances. In each Sudoku puzzle you need to have nine of each number within each region, each column and/or each row. So you can often start to see which number is most plentiful amongst the givens.

For example, if you scan over the puzzle and see 5 sevens amongst the givens, then you can start to see if you can use logic to find out where the other 4 sevens should be.