It’s all methodical and there should be no guesswork involved.
First I do all the obvious singles, where there’s only one cell a number can be in. There are usually at least two or three of those, or more in easier puzzles. The early numbers you fill in often lead to more singles.
Next, I look at rows, columns or boxes with 4 or fewer empty cells and I write in pencil at the top of the cell, all the possible options. Look for pairs: if you have two cells with a pair of candidates you can eliminate those two candidates from all the other cells in that row/column/box. You can do the same with triples, and even quads in a more complex puzzle.
At this stage you should fill in the candidates for all the remaining empty cells.
Look for locked candidates next. This means that if the only occurrences of a particular candidate is within the same row/column AND the same box, then you can eliminate that number from the rest of the box.
These three strategies should solve most puzzles except the most difficult; in those cases you’re going to need to start looking for things like X-wings and Y-wings, which is far too complicated to explain here without diagrams. There are plenty of strategy guides on the internet which explain them.