as mentioned before, you want to find the studs. tapping with your finger to find the solid sounding spot and then using a small nail to verify works great. from there you can typically measure over to what your standard spacing is in your country. or check manually as you only have a few holes. a studfinder is easy, but not required.
mounting with in-wall mounts, be it any of these
http/image.made-in-china.com/2f0j00VMItQaRyVOgp/Wall-Plugs-Nylon-Plugs-Standard-Anchor-Series-All-Fastener-Series.jpg do certainly work for light weight and in some scenarios however are weak. in your case the vertical height between the fasteners is not very much so it will put lots of force on the fasteners and pull them out or crack the wallboard. fasteners such as linked work great in cases where the vertical distance between screws is high compared to the extension of the shelf from the wall (such as if you had a 6 inch shelf but had 6 inches or more vertical between the two screws) or in cases where you simply hang something directly perpendicular (straight down for example when you drill into your wall). one screw into a stud is as strong as 5-6 such fasteners as well, so you may want to keep that in mind.
while outlets for the most part are secured to studs do not assume this without testing. some do get placed just hanging onto the wallboard (installed after wall is already up).
the ikea shelf you linked.. yeah that one is a prime example of something that will have alot of force on the screws given its small vertical distance. too much weight and it will pull the top screws right out. do yourself a favor and put them into studs.