home alarm system using microcontroller

This is far too complex for you. Start by flashing a LED with 2 transistors.

 

What kind of experience do you have with microcontroller and programming so far?

Using something like a PIC or AVR will tend to give you smaller finished products and in my opinion provide more robust and professional results compared to using an Arduino or Raspberry PI.
That said... they are harder for beginners to work with... both electrically and programming wise.

Using an Arduino, you will find that the boards cost a little more than the above, and can be a little bigger, but the community support and parts available target beginners. You can get all sorts of 'Arduino' compatible gear to do almost anything.
They are a little more expensive, but you'll find more support, and programming will be slightly easier to manage.

Now.. the last item is a Raspberry Pi, or BeagleBone... or any other "SoC". These typically run an operating system, most commonly Linux and are usually much easier to program because of a much expanded number of options for programming languages. You can program in C, Java, Shell scripts, etc... ANYTHING that the OS supports can run your program. These options are also more expensive and larger, but also typically have more power than the above options.


I would suggest a RaspberryPi, as I feel it would be easier for you to build your system with... Some major limitations of this is that they are typically much slower at checking the status of the input output pins, and usually lack analogue inputs... What this means for you is that it will be a little trickier to monitor temperature, or to output to addressable LEDs or anything else that requires very precise timing of the pins. This means very little to you, but the advantages far outweigh it.
It also has USB inputs, Networking, and audio support so you can let your imagination go a little wild.
You can use 'internet' calling or other online services to send a text or call to any phone number you desire without extra hardware. You could run the audio from the Pi to an amp and speaker so you can 'talk' to give a better impression that someone is home. The usb inputs can be used with a USB hub if you want to run a couple USB web-cams and because it's network ready, remote access will be incredibly easy to setup and you can retain a secure connection without having to worry about all the fine grain communication methods required to do the same with a microcontroller.

 

Don't fret. I learn a lot more, a lot quicker when I stretch my abilities and try something above my means.
This project is going to push you rather hard, but I encourage you to push through it.
Take the project apart and think about the individual 'functions' you want, and work on each function at a time and you will be just fine