Hi everyone,
I'm new to the Arduino community and recently started learning about microcontrollers and DIY electronics projects.
I would love to know:
Thanks in advance for your suggestions!
I started with Arduino by byuing Starter Kit and doing all projects there. (It took like one ot two afternoons.)
The starting projects there are really easy, maybe even little childish (let's make a rocket = 3 led blinking, if I remember correctly), but they show soma main principles, are short and fast introduce to some possibilities.
Mainly, after finishing that, I did know, that I can easy use all mentioned sensors and other pieces (LEDs, motors, servos ...) in my future project and simply copy part of Starter Kit to get the access. Also it give some idea, what kind of projects would be easy with Arduino.
( Set UV LEDs to harden cover on my PCB for dedicated time = easy. Fully working Death Star = not easy. Etc. etc.)
I was experienced in programming at the time, did not know, that Arduino just use C++ (avr-gcc), and had no experience with embeded MCUs. Looking back I would not choose different approach.
Yes, the potencial of Starter Kit was used really soon, but it was really good and fast start. So for me it were well spend money. Well I am "rich" so much, that the price was no problem for me and I used some parts from the starter Kit many years after that.
I think, I started around 2013.03, buying the Starter Kit. Last time I used the included Arduino Uno (THT MCU in socket) was yesterday, just to demonstrate some ASM features 
None. I started with barebone Atmega when I used AVR the first time (which was not the first time using µC either, so ...)
Maybe you should ask: "why are you guys still using Arduino?"
I already had a bunch of parts and experience with other processors, so I didn't go the starter kit route. If you are starting from scratch I would recommend a starter kit. There are a wide range of components and prices. I would recommend the low to medium price range. The more expensive ones have more exotic parts, but if you don't have an interest in them it is a waste of time and money. If you later change your mind you can always get them separately.
As far as parts that I would look for in a kit
LEDs - blinking an LED is one of the first things to try
potentiometer - generate a variable voltage to sense for controlling other parts
Light sensor(photo resistor) - sense an easily changed external condition
Temperature sensor - again sense an external input. There are two types, digital such as the DS18B20 or analog such as the LM34,LM35 or TMP37. The analog ones operate similar to the light sensor, the DS18B20 operates differently and will involve the use of a library.
Small motor/servo - got some motion going.
LCD character display - display somewhere other than on your PC. Some have an I2C "packpack" these have a different interface using fewer wires.
Whatever else looks interesting/amusing.
I started with the Arduino Cookbook, the Arduino IDE, two Uno boards, and a well-stocked electronics lab. That gave me enough to learn C++ (which was a challenge coming from an assembler background) and begin building projects.
I especially like the Nano because I can treat it much like a bare microcontroller, but without the hassle of crystals, programming hardware, and other support circuitry. Just plug it in and go. I've completed several projects where the Nano simply plugs into my custom board and serves as the controller.
My project boards eliminate the need for many external modules by integrating features such as the RTC, SEPIC converter, and power outputs directly onto the board. That keeps the design compact, reliable, and easy to assemble.
The default advice for beginners is to get an Uno. This is unfortunate, because usually it's not a good choice.
The Uno is a special design, or form-factor, for use with shields. Shields are ready-made circuits on PCBs which have the same shape as Uno and fit on top of the Uno.
If a shield exists which contains the circuit you need for your project, then great, go ahead and buy the shield and an Uno.
But usually beginners want to build a variety of circuits to learn about different components and gain experience. So a breadboard to build the circuits on is the obvious choice.
Unfortunately, Uno is not exactly breadboard compatible. You can use it with breadboards, but you can't plug the Uno into the breadboard and are forced to use a mess of unreliable Dupont wires to connect the Uno to the breadboard. The result is fragile and difficult to find any faults in.
So I would also recommend a classic Nano V3. These really are breadboard compatible because they can be plugged directly into the breadboard. And Nano is the closest thing to an Uno (without the unfortunate form-factor).
I would agree with @PaulRB on the Uno.
It gets it popularity from long life and the sheer number of references to it.
It is what it is. A development board with all the bits and pieces to get started and do some prototyping.
That is, until you hit the header pin spacing problem.
Why oh why wasn't that sorted on day one?
Nowadays, I don't buy Uno's apart from departures into some other area like WiFi, but then, only for learning.
The Nano in all it's types is my favourite, small, easy pins, and cheap enough to use one per project and incorporate into finished equipment.
Of course, I'm indebted for the Uno for making my first steps into the world of microcontrollers, and not least, the whole Arduino concept for making the microcontroller accessible for complete beginners.
To be it more "human", that is why ...
... when I put some finishing touches I managed to get the spacing wrong...
We had 5 minutes before the deadline to go into production, the PCB guy was on the phone saying "send it now or it goes to next week" and we didn't have a name yet... they I said let's call it Arduino like a bar we used to go...there wasn't much time to think.
Then I'm fond of the mistake... it makes it not perfect therefore more human... Yes...it could have been all perfectly optimised... but if engineers ran the world it would be an unbearable place where to live
From an analytical perspective, I'd first browse through the examples included with the Arduino IDE. They cover many of the basic building blocks, and combining them often leads to surprisingly interesting project ideas.