No matter how experienced you are, a written pattern will eventually give you a line that makes no sense. This does not only happen to beginners. I have been crocheting and knitting since I was a child and I still run into it.
Start with the abbreviations
If you are new, make sure you understand the shorthand before anything else. Most patterns list the abbreviations they use, and this site keeps hook size and abbreviation charts for the ones that get assumed.
Check which convention the pattern follows, too. US and UK crochet terms use the same words for different stitches, and a pattern in the other convention will produce a fabric that looks wrong for reasons that have nothing to do with your technique.
The problem is usually further in
Trouble rarely appears in the first few rows. It turns up in a row or round some way into the pattern, once the shaping has started and the instruction has become dense.
When you hit one, do this in order.
Read the line again, slowly. Most confusion comes from reading past a comma or missing a bracket, not from the instruction being genuinely ambiguous.
Work it in your head. Imagine each step without picking up the hook. This catches misreadings that your hands would otherwise commit to.
Then work it for real. More often than not, people are overlooking a portion of the line rather than misunderstanding it.
Brackets and repeats
Brackets are where most patterns lose people. (dc, ch 2, dc) in next st means all three of those go into the same stitch. [dc in next 3 sts] 4 times means work the bracketed part four times in total, not four times in addition to the first.
An asterisk usually marks a point to return to: *dc in next st, ch 1; repeat from * across means keep alternating until the row ends.
Read the punctuation as carefully as the words. It is doing real work.
Check the stitch count
Most patterns give a stitch count at the end of a row or round, usually in brackets. Count against it every time.
If your count is off by one or two, the mistake is in that row and you can fix it now. Discover it ten rows later and you are unpicking an evening’s work. This one habit prevents more frustration than any other.
When it still will not work
Put it down and come back to it. A line that is impenetrable at eleven at night is often obvious the next morning.
If it still makes no sense, look for other people who have made the pattern. On the pattern databases, someone has usually hit the same line and written down what it means.