Just got my first guitar!

I love the ocean, and this blue with the maple fretboard just reminds me of the ocean and the sand, so i had to get it.

Would appreciate some tips about how to place fingers during chords like G# minor nineth, where three of your fingers are on one fret, or chords where fingers are placed with skipping frets inbetween. Is it okay to mute a string that isnt being fretted? Or should all strings play when i strum a chord, because i keep accidentally muting some strings. Thanks in advance.