What’s the first thing to do after installing a new computer?

My answer is- Promptly plan and install a BACKUP solution.  Computers are so handy and many users just use it daily without thinking that they may fail.  Yes, they do!  You may notice that I use ‘may fail’, not ‘might’.  They do breakdown like any other machines.  If the harddisk fails/crashes which is more likely (most harddisk has mechanical parts and it has s a finite life from the thousands of accesses each day/week) than the other hardware in your computer, then all your data may be lost – a rather catastrophic scenario!  This is NOT rare, so if you don’t have a plan for backup, you’re likely hit with this.  Don’t be complacent with the “lifetime guarantee” provided by the harddisk manufacturers, they only guarantee the hardware and may replace/repair it should it fail, NOT the data within.  So make sure that you install your backup solution right away after setting up your new computer.  And remember, backup means alternative solutions when your computer fails, so it has to be physically elsewhere to be able to provide the alternative, not within the same harddisk in your computer.  Below are something you may consider:-

• Backup should be alternative and located elsewhere – it can be an external harddisk (or USB finger (flash drive)- see below), or off site (even better)- stored in the ‘cloud’ would be a handy solution for ordinary users nowadays.  Clouds are basically large harddisks (data centres) managed by service providers (e.g. icloud by Apple, Dropbox, Google Drive, Skydrive, etc.  Many of these cloud services are free for the first few GB, but you have to pay for additional storage.  Clouds have the advantage of being accessible from anywhere by any device so long you have internet access.  But remember, cloud storage access (upload and download) requires internet/wifi and is limited by the your access speed.  It’s usually ok for normal access for a few files, but the initial backup of your complete data in your computer will take days, weeks or may be months!
• USB finger storage- while 512GB USB fingers are available, they are still relatively expensive, but 256GB and 128GB fingers are getting more popular costing from only around HK$400 (Feb 2015) which I’m sure the price would continue to drop.  Hence USB fingers are becoming real handy options for carrying data around.  Get an USB 3.0 one for faster data transfer.
• see this for format options for your external storage device.
•  Backup apps – now that you decide on what ‘hardware’ solutions to use, you need a backup apps.  You can of course do it manually every now and then by copying across, but that’s very inefficient and if you only remember to do manual backup very infrequently, then you are at risk of loosing everything in between.  Hence a automatic scheduled backup solution supported by a backup app is the solution.  E.g. Mac provide an out-of-the-box solution of Time Machine, you only need to set up which harddisk to use for your TM and options if any to exclude some specific folders/files, then TM would do the backup automatically in the background and provide you with backups as frequent as each hour.  For Windows, you can refer to this good review, I’ve tried CrashPlan which is easy to use, but as with every ‘free’ software, you need to read carefully the limitations/conditions for free usage.