It's quite a short answer as you don't have many skills at level 15.
First off: There is no bonus threat anymore. It's now passive for all tank specializations. Monks used to have Dizzying Haze for that, but it was removed in Legion.
- Whenever it's ready use Keg Smash
- Use Chi Burst or Chi Wave if you used one of these as level 15 talent
- When Keg Smash and your lvl 15 skill are not up use Blackout Strike
- When they all are not up or you're out of energy use Tiger Palm
Your taunt is Provoke. Use that on bosses or other mobs that are out of melee range. Use Roll to get to the mobs faster when your taunt is on cooldown or you need it for another mob.
With higher level it will be easier to keep aggro. You'll get more skills as you level up.
- Crackling Jade Lightning at lvl 18 is a good range skill
- Chi Torpedo (lvl 30 talent) is quite useful in dungeons
- Breath of Fire at lvl 40 is a decent AoE skill that helps a lot keeping mob groups aggro
Especially in low level dungeons the damage dealers are often reckless and pull too many mobs or just attack the boss without you being ready. If they die, it's their own fault (and as a tank I might just then take a 10 seconds coffee break when someone keeps pulling mobs without my permission).