There's no secret to get motivation to start, or to continue playing. I started to play drums around my 12 years. I started the guitar around my 13. I tried a lot of other instruments (violin, keyboard, trumpet, else...). Almost 20 years after, I'm still playing drum, guitar, bass, and keyboard on a daily basis. I even bought an electronic drum kit last month )
Most of the time, the motivation come when I listen to music, or watching movie/serie/anime. You know, I'm like "Hoo well, this drum rhythm/fill is so nice, let's try to play it!" or "Gosh, this guitar riff is f*cking good, can I nail it?" , and "PAF!*, I found myself plugin my rig and playin'.
Sometimes I just got some inspiration while daydreamin...
About the exercises (cause you know that you have to do sometimes exercises, even if it's not the funniest thing...), I see them as challenges, and I do them everyday. I give myself some objectives and rewards. Example given, when I work my double/triple snare rolls, I give me objectives and duration, let's say "holding my triple snare roll 1 full minute". After that, I allow myself to play something I really like to (a band I really like, some good blast beat, double bass pedal beats, etc.). But you need to do your exercises, it's a mental discipline to establish, some routine.
All that is more about keeping it up than starting an instrument, but these tips are adaptable I think.
No secrets, only discipline. Ho, and the last but not the least advice : "Do or do not, there's no try". Be sure of yourself, of you capacities. And play.