First I want you to read Bruce Schneier’s excellent article How to remain secure against NSA surveillance. Done? Ok.
It’s most important to understand that it’s not simple at all to find security when it comes to the Military Industrial Complex and their cyber division.
Second, in case you’re a already a target of the NSA/GCHQ or whatever State sponsored spy agency you have to learn that it’s impossible to stay secure against online surveillance. Whatever whoever says – no change for you. Sorry!
Now we can start with Your computer: Use cheap hardware and get rid of all the Microsoft and Apple spyware operating systems. Use Gnu/Linux! (Yes, I know this will not help against hardware backdoors in Intel or AMD chips – there are alternatives as well. Or you may use a device which isn’t connected to your person.) Anyhow go here and choose your OS.
Your files: Encrypt all your files. Your harddisks. Don’t use commercial encryption software, use well tested Open Source Software. (example: Don’t use PGP – grab GnuPG)
If there’s senitive information – don’t you dare store it on a computer connected to the internet.
Never ever! Use USB sticks to transfer files. Crash them from time to time and buy new ones.
Your communication: Hide and Encrypt it! All of it. Use Virtual Maschines, different anonym VPN provider, Tor and whatever is within reach. Use TLS and IPsec. Encryped communication is harder to read anyhow. Again avoid commercial software here! Consider to use unbreakable OTPs – Educate Yourself here.
Important reminder: Your Mobile Phone, Smartphone ecc. is the best ever invented surveillance device and it’s constantly tracking your location as well. It’s always a good idea to leave it at home or to have it disassembled (battery, SIM-card). Use prepaid phones!
Very important reminder: Get of the grid from time to time or forever!
Last reminder: None of the above is foolproofed. It’s always better to hide the fact that you’ve something to hide and it’s always better not to speak about secrets. <+ re-read that!