Flying indoors is a terrible idea as others have said. I have flown other drones so I can fly in basic atti mode, plus I have prop guards if I wanted them, and I _still_ wouldn't fly indoors for fun/practise but only if I really wanted to get a shot inside a building. I have done that once, everything was fine, but it requires care and particularly requires RTH set to hover.
Your garden should be OK, but frankly if this is your first quadcopter I'd actually go to a much bigger space first, and fly initially in tripod mode as others suggested, because what you really want to do is build some muscle memory and reflexes in terms of what the sticks do, particularly when the drone is facing either towards you or sideways - it's easy to intuitively move the right stick correctly when the quad is facing "forwards" away from you, because left is left, forward is forward etc, but once those directions are no longer referenced to you but to the mavic, add a dose of panic and haste and you could make a wrong control input.
None of this is intended to overstate the difficulty - it's very easy to fly the mavic, and within a couple of hours you'll be much more comfortable, but for the very first couple of flights it would be the lowest stress situation and the most conducive to learning to be well away from obstacles in a large clear area where you can try it out without risk or worry.
Edit: another thing, based on something else another new drone user posted - don't be afraid to climb to a decent height either (wind permitting). It's safer to be up above obstacles - I know it probably _feels_ riskier, but that's not rational - if it crashes from 10ft or crashes from 100ft you'll probably break it in either case, but the main difference is that if there are any obstacles around you're much less likely to crash from 100' than 10'.