I assume you are asking how to move a Family Tree Maker File on the old computer, to the new computer. If so, make sure you back up the file on the old computer.

Copy the files associated with that tree to the new computer. You could use a thumbdrive, LAN, or what every you have.

You said OLD computer, I assumed you had Family Tree Maker maker on the old computer. If so you need to know what version of Family Tree Maker. How you open it on the new computer depends on what version you had. Note: There are basically tree groups of old versions. The first are version earlier than about 8. Those version around Version 16, and then the version that have code and a database structure similar to the current versions. These are know by a year designation 2008, 2009, etc. I believe I am safe in saying you can import them all into the current version 2014, but each is done differently.

If the file on the old computer, has been synced to Ancestry, I believe you can download the Ancestry file into the the new computer/program. I am unsure of this. There would then be no need to worry what is on the old computer.