It is probably best if you follow the instruction on the Ubuntu, or other OS, site as they can vary a little but it is not complicated. There is also a useful tool called unetbootin: https://unetbootin.github.io/
If you are installing on a machine that is running a later version of Windows 7 or above you will need to disable the secure UEFI boot, don't worry about this, it is a useless and annoying Windows thing that serves no useful purpose.
Linux is easy to use but rather than me waffle on, it is probably best to see how you get on with these sites and then come back if you have any problems or more questions.
Never used it but would expect you to need to input your wireless password during set up. If it is for the WiFi you already have you should be able to find it with your current OS. In most it is as simple as clicking on "show password" or something like that in the WiFi section. Not sure about Windows as I do not use it.
What do you mean for the wifi password?
You mean for your router, then unless you have changed it yourself it will be on the sticker attached to the router.
If you changed your wifi password and have nor forgot it, then you can simply reset your router and it will be all the default passwords that are on the sticker on the router.
I have to cringe every time this sort of thread comes up. From reddit to here, different forum same thread.
Firstly, you really don't want to touch Kali (the updated version of Backtrack) with a barge pole. Sure, its nice to have a single download for a live environment with tons penetration/security testing tools. But Kali isn't a distro designed for everyday usage. It is much better to download the particular tool you need and install it on a proper distro.
Secondly, just because Kali has certain tools installed doesn't mean your system is configured to use them. Downloading something like Fern or Aircrack or whatever is pretty pointless if you haven't got your wireless drivers set up properly. And that's assuming that your wireless hardware is compatible with your software suite in the first place. Intel chips, for example, are pretty common and tend to work well out of the box for normal usage. But you have no guarantee those chips are capable of working with your software. Getting the needed drivers working properly in a sensible distro is hard enough, but trying to configure those under something like Kali is just asking for extra trouble.
Thirdly, all of this is completely pointless if you don't have at least some understanding of the network protocols involved. While GUI software is easier to use it also tends to be the least useful when it comes to finicky stuff like this. Fern, for example, has a fraction of the power of pure Aircrack. But trying to use something like Aircrack when you haven't a clue about what it is doing under the hood is a complete waste of time.
Moral of the story? You really need to read up A LOT on stuff like this before you can even attempt it.