MyBB Newbie Help

Hello to everyone,
I recently purchased my first site, and create a forum for it. Thing is before I buy some premium themes, I wanted to tryout the ripped version of the majestic (which is pretty good theme),
but I literally have no knowledge about mybb forums, so I am here to give a shot. I checked the site content but couldn't find a answer to my solution. If you guys(and gals) help me that would be terrific,
so my questions are simple really (I dont even know this is the good place to post this thread but forgive my mistake if it's not) :
1) How can I edit my theme?(if we talk about majestic 1.8 theme for example, how can I edit logo, links, translate some of the buttons to my original language and so on)
2) What's the "must" plugins for fast and productive forum?
3) And really any kind of information about mybb .. since I would love to hear anything and try to learn.
I would put more questions if I counter them so thank you for all of your answers .

Hi !
To edit logo, you need to go to your theme settings
To edit texts & other stuffs of the forum display, edit the templates of your theme, find the words you want to edit
For plugins, use registrations questions (to avoid bots), to avoid leech you can use my ratio system : http://nulledbb.com/Thread-Ratio-System-1-0 with Hide until reply plugin, use Google SEO & Spider Bots (for SEO), use Admin CP Honeypot (to avoid hackers).

Thank you for this amazing answer, that was very helpfull !

Now I have problem with the textbox (Iwouldn't say problem but, it just look like it doesnt belong there),

4)How can I modify this textbox ?

5)How can I edit the threadicons ?, and how can I find icons suitable, as you can see it just doesn't fit the theme.

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Anyone can Help me on this subject ?