Taking the DIY route has pros and cons.
For:
More choice of parts; you can fully customise the build as you see fit and use any supplier you wish.
Lower cost.
You can spread the cost by making a parts list and purchasing over a period to suit you rather than paying up front.
Against:
Nerves; Many who are more than capable of assembling a PC choose not to do so because they're worried about damaging a component or making a mistake.
No warranty; each part will carry the warranty applicable to the country it is purchased in but you will be responsible for taking any actions needed to make a claim, a system purchased pre built will carry an overall warranty that many find worth the extra cost.
Credit is available; many companies offer credit terms, allowing you to spread the cost.
Most DIY build guides miss out on some of the little details that make the first build go so much more smoothly:
It's not particularly hard but there is some potential issues to bear in mind:
Static is an overrated risk, touching a radiator pipe is a good way to keep it at bay.
You'll need a few tools to make the build go smoothly: A good quality, magnetic Phillips screwdriver, either a pair of pliers or a suitable socket wrench to tighten the motherboard standoffs, possibly a pair of tweezers and a magnifying glass to help with the front panel cables ( some cases use tiny single cables ).
Work on a flat, stable and well lit surface that is free of clutter and has enough room around it to store the parts and empty boxes.
ALL the power cables will only go in one way, if one seems too hard to insert, check you have it correctly aligned.
That big 24 pin motherboard connector is always tight, lubricate the plug with a little petroleum jelly before trying to push it home with one hand while supporting the underside of the motherboard with the fingertips of your other hand.
It's easier to install to motherboard with the CPU cooler attached, it gives far better control to have something chunky to hold on to. Also lower it in at an angle so you can slide it into the rear IO shield before lowering it fully down onto the standoffs. In most cases that IO shield has a number of springy fingers on it that will push the motherboard a little ' forwards ' and out of alignment with the standoffs, attach a mounting screw to the 'driver, gently push the motherboard into alignment and start, but don't tighten that screw, repeat until all the mounting screws are started THEN tighten them firmly down.
Check the case and case manual, there's a 4 or 8 pin connector block on the motherboard, usually top left, and it's not always possible to route that cable from the power supply, behind the motherboard tray and out the top of it with the motherboard installed as the motherboard can block the exit hole, if you're in any doubt about this, install the power supply first, then route the 4/8 pin EPS cable then install the motherboard.
Sellotape and/or elastic bands can be great friends, use then to keep stray cables out of the way, especially when installing the motherboard.
General tips:
Allow plenty of time, don't rush, turn off your mobile and don't get distracted, if you have to move away from the build, finish the task you're doing then move away, never leave a task halfway through, it's an open invitation to Murphys Law to step in.
DONT'T PANIC! If it doesn't start up first time you've probably made one of the embarrassing errors we have all made in the past so start with the basics: Is the power lead plugged in? Is the socket turned on? Is the PSU switched on? If yes, power down, disconnect the power cord wait a minuet to allow the motherboard to discharge then check the cables are fully home and in their correct locations.
Finally: READ THE MANUALS! It is essential for your first build to read and understand these documents.