When naming carboxylic acid you first identify the longest continuous chain of carbon atoms that contains the carboxyl group (-CO2H).
In the following example, there is one carbon. So the corresponding alkane would be methane. Replace the "e" in methane with "oic acid" to get the name of the carboxylic acid, so methane becomes methanoic acid. Methanoic acid also has the common name formic acid. I've never heard anyone call it methanoic acid.
Add another carbon and you get ethanoic acid (common name acetic acid).
If there are other substituents (groups), you precede them with the number indicating the position on the longest chain. Numbering starts at the carboxyl carbon.
Common Named Carboxylic Acids
Some carboxylic acids have common names you should become familiar with. Pyruvic, lactic, and citric acid have importance in biochemistry.