Hey Antonio
Phew lots of questions!
In general your questions mainly relate to quality. Point to note is that the microphone in that video is not a shure sm58a beta.
You can just use your PC mic input, but generally the ADC (analogue to digital converters) on most basic PC sound cards are not that great, and is a big part of the need for better quality audio hardware for your computer. The other reason is that most PC sound cards in off-the-shelf computers are on the motherboard and tend to be more prone to higher noise levels due to a number of reasons. So, a range of sound cards with or without external boxes, or purely external sound boxes are available that improve noise performance including good quality ADCs. Prices range from about $80 - $1,500+ (Read that as £s in the uk as the actual numbers are comparable). Interfaces range from old USB 1 to modern USB, or FireWire. There are other older formats too. I would suggest USB 2 or newer.
I would suggest an entry level system. More expensive does equal more growing room and features, but they usually also involve a larger learning curve.
Most come with a copy of the lite version of cubase, cubasis, or alternatives like cakewalk sonar, so no need to go buying a full version until you know your sis will stick will it, same goes for expensive hardware.
Software VST effects (plug in effects and audio processors) can be bought, but for your immediate needs some will come with it and others are available for free.
A pop blast screen can be easily made from a an old CD or DVD multipack spindle cover and tights. Enough to get you started there.
So that leaves your hardware interface to plug your mic or instruments into. This comes down to your budget.
Can you give me an idea of budget? There are loads of low end to mid end interfaces available, so budget would help narrow it down. Additionally, are there other needs or the interface? Ie do you play a synth keyboard, or your sis? A guitar? Pretty well all will cater for instrument, vocals etc but the better an idea we get or needs and what you may oreads have the better.
