This could easily be a book-length topic.
You need to solve several distinct problems. I strongly urge you to work them out one at a time!
You need some kind of structure for records in the file. Maybe delimited fields, fixed length fields, XML tags, serialized objects or many other choices.
You need some kind of structure for records in memory. A simple
Java object with 21 fields or a Map would work.
You need to read the file structure and construct the in-memory structure. And probably write the in-memory data back to disk.
10 mb is not all that big, so reading it all into memory is not out of the question. A List of some kind could probably hold it all. Whether you read it all into memory or read one line at a time process it and throw it away you need some kind of search. The details will depend on your storage.
Any of those problems sound like something you could take a shot at? Again, pick ONE and show us how you think you'd do it.
Oh, and think about a database. This is exactly what they are there for!
