This would probably get a few more responses in the Server forum; the purpose of the How-to/FAQs forum is to allow people to post their own tips, rather than asking questions. Anyway, to answer your question, you'll first need to decide on which FTP server you would like to use. In the past I've found
PureFTPD easier to set up initially, though
ProFTPD seems to have quite a few features you may like. Obviously there are others to consider too, but at this stage there wouldn't be much sense in overloading you with options.
Once you've decided on the server you want to use, use APT to install it (e.g. `apt-get install proftpd`or `apt-get install pure-ftpd`; if you want something else, use the
Ubuntu package search). Once you've got it installed, it's generally a good idea to get into the configuration files to configure the server to your specific needs. I don't know what those needs are, so you can ask more specific questions if configuring the server proves challenging.