A tuner is easily going to be one of the most important tools you can buy for your instrument. Keeping your strings in tune is the easiest way to have your instrument sounding the best that it absolutely can. Personally I prefer the tuners that simply clip on to the headstock, and you can easily pick one of these up for a relatively cheap price, usually between 15 to 20 euros.
A great example of a decent and cheap tuner is the clip on Korg PC-1
A capo is the easiest way to get your guitar to play in different a key. It clamps on to the neck and effectively shortens the length of the guitar strings, making the guitar sound higher in pitch. Capos have a couple different styles, either being a spring clamp or a screw-on; both types will typically run from 15 to 25 euros.
Depending on your playing style, the right picks or picks can make all the difference in your style. The traditional player will use a single flat pick, and more folk or classical style playing can use picks on each finger. Picks will usually have different thicknesses and materials for the desired sound they can create, and it can take some experimenting to figure out what suits you best. Harder materials will generally create a louder and crisper sound, but lighter materials will be better for strumming chords, for example.
A good strap is a must have if you ever plan on performing in public, or if you ever want to play standing up. With straps, comfort is the most important factor, because the more comfortable you feel, the better you will play. Straps are also available in a wide range of colors and styles, so you have the opportunity to customize your look as much as want.
Metronomes may seem unnecessary, but they are so important for practice. Learning to play steady at the correct tempo is something a lot of new musicians forget to practice, so do yourself a favor and get a metronome to stay ahead. Good habits make for good performance.
If you ever plan on traveling with your guitar, you are going to want to get yourself a good quality case for transportation. Hard cases are a great option for extreme durability, extensive travel, and storage in general. Hard cases are usually made from a thick plastic on the outer shell, with a soft textured interior. Soft cases, on the other hand, are usually made with minimal padding because they are meant to be carried along for light travel. Soft cases are quite light in weight and will generally be a bit cheaper than hard cases.
The Chord Solid Foam Case is an excellent choice, it's ultra lightweitght but it is also extremely durable and can keep your guitar safe.