Surround Sound, 2.1 or classic stereo?

It depends of what you most often will play on your system. If you listen mainly music probably it will be better to choose Stereo or 2.1 system. Most of the musical recordings are not mastered for surround sound. If you will watch movies - it's clear: surround sound.
2.1 system is very good alternative to classic stereo system. The sound produced by 2 small bookshelf loudspeakers and relatively big subwoofer may be quite similar compared with two big floorstanding loudspeakers. This gives flexibility in the room arrangement and sometimes there is price advantage.

What is 8 Ohms or 4 Ohms? Is it important?

Yes it is. This is the impedance of the loudspeaker or with other words what load to the amplifier makes the loudspeaker. Bigger digits mean less load and vice versa. The modern amplifiers operate usually between 4 and 8 Ohms. It is necessary to check this in the amplifier's characteristics. Do not connect loudspeakers with lower impedance than the amplifier can handle. It is very probable to blow up the amplifier.

Which type of the loudspeaker's power to rely on?

The RMS power is accepted as a standard. You need it to choose the right amplifier. Sometimes the manufacturer of the loudspeakers recommends the power of the amplifier which will drive the loudspeakers optimally.

Is the frequency response the main thing to consider?

No. This is very important characteristic, but it is most often manipulated than all others. Look for the SPL graphics in the owner's manual. If there is not such one, look is there shown the bandwidth of the measurement. If it is +/- 3 dB it is OK, the frequency characteristics are true, but if +/- 20 dB or at all it is not mentioned, the information has no sense. Do not expect miracles too. There are laws of physics against which we cannot go. If a small bookshelf loudspeaker has claimed low frequency 25 Hz, forget it. And the golden rule: listen! Believe mainly on your ears, not on papers. Your ears hardly can be manipulated.

Where to place the loudspeakers?

Most often - where your lady says. But seriously: try they to be symmetrically according the sweet spot of your listening room and if possible at the ear's height. Usually the smaller loudspeakers need to be closer to you and the distance between them have to be less. But again: listen and experiment.

Are the cables important?

Yes. It is good to have as better cables as you can afford.

The more heavier loudspeaker produces better sound - true?

It is connected with the resonance and interference. Usually the heavier materials have less of them and this is important for the sound.

Why the loudspeakers sound in other way in my room compared with the showroom?

It is connected with the resonance and interference. Usually the heavier Because of the room acoustics and of course all parts of your system. Usually there are not 2 rooms with equal acoustics as well as probably for the demonstration were used other sound sources, amps, cables etc.