I've been teaching piano and voice lessons for fifteen years, and right now I'm the director of music at a church. Here are my suggestions:
1. Play songs that you love, early and often.
2. Play regularly; nothing beats hands-on practice.
3. Be prepared for the jumps in your abilities after a good night of sleep; your brain will often work things out while you're snoring.
4. There's nothing linear about learning to play an instrument; expect cycles, plateaus, and sudden spikes in your abilities.
5. Listen to music you love from a variety of genres and discover new musicians who inspire you.
6. Definitely look for a teacher who tickles you, intrigues you, inspires you, and supports you.
7. It's better to practice a little bit every day (for even five minutes) than it is to ignore your music all week and then binge for four hours on Saturday--that's a recipe for burn-out.
8. Figure out which parts of mandolin playing really light you up. Do you like strumming? Finger picking? Classical guitar-like things? Gnarly jazz solos? Pay special attention to what you love to do and do more of that. (Once you're there--I realize that you're just starting.)
9. If you like
permaculture, you might love the structures in music theory. Once you get beyond key signatures, sharps and flats, and note names, you can learn wonderful forms and patterns in music that are wonderfully synergistic and puzzle-y.
10. Don't put too much pressure on yourself to be perfect. Be light in your being and try not to stress out over missed notes or rhythms.
11. You can have the most fun when you're playing music when you're playing it with other people, and to that end, being able to play the right chord at the right time is a golden skill. If you learn how to play along with recordings and keep up, neither drifting too slowly or rushing too quickly, you become an asset to a group. To this end, YouTube can be very useful. It can also be useful to start playing with other (supportive, kind) people as soon as you can in duos or trios. Just friendly living-room musical puttering.
12. If it hurts you physically, first rest --- and then adjust your ergonomics, your posture, or your hand position until it doesn't hurt.
13. Consistently showing up to play the mandolin, year in and year out, will yield more consistent success than having talent without follow-through. Show up and try, show up and try, show up and try, show up and try, show up and try, show up and try...
14. Get familiar with the basic routine of caring for your instrument and know how to tune it and restring it as necessary.
I'm pretty excited that you're starting this musical journey. Keep us posted about how it's going for you, if you'd like!
**edited to be talking about the correct instrument. =P