At 50% wear the bar at the bottom will go yellow, but the gear is still fine. At 30% there will be a yellow border around the item icon. At this point the benefit from the item will start to reduce. At 20% the border will turn red and the benefit will be drastically reduced (note you won't get a visible indication of reduced stats on your attributes but you may take more damage). So the "omg you need to repair" is 20% or less.
To get your gear repaired by someone else, simply go to the item in your inventory and then on the pop-up menu choose "seek repairs". You can then set the reward. However much you choose to put there is up to you, but a rule of thumb it is put the Grade of the dark matter needed x1000 auro, or if you have the dark matter in your inventory you can offer it as the reward. Once you have it set up, you will have a hammer icon next to your name so people will know to check your bazzar for repairs. If you stand in a place with much traffic or near the Repair NPC in town the repair should be fairly quick.