If it is the typical .sql file, then the file is a simple text file which contains the SQL commands to make tables and enter data into the database. If you are lucky, all you have to do is open the file in a Reader, read the lines into character arrays then execute them on the SQLite database. There are, however, things to consider:
1) Where the .sql file came. Is it a trusted source? Should you have to worry about it destroying data? Injecting bad things? Putting bad/inconsistent data in the database?
2) What the target database is. Is the SQL included in the .sql file general or does it have DB engine specific commands?
You should probably open the .sql file and read it to understand what it will do to make sure the above two issues aren't a problem. If they are, then you have a ton more work to do.