What should I do with worthless dwellers?
Some of my dwellers are not good at anything, for example, -1 cooking -2 water -3 electrical etc.
What should I do with them if they are not going to be producing any resorces? I can't just let them sit outside, because that will annoy me.
You have several options for low-level vault dwellers.
Explorers
Firstly, you could use them as explorers. Send them into the Wasteland, and see what happens. You never know, they may just happen to find some good items, out there. You can look at your explorers while they are out in the Wasteland, and get feedback on any items they have picked up, too. Recall them, if they find items you would like. They will not take any damage while returning. Otherwise, just leave them out there. Eventually, they will die.
Breeders
You could always use your extra dwellers in a breeding program. In my experience, parent stats do not make a big impact on the child stats. I have had good stats come out of two low-level parents, and I have had mediocre stats come out of high-level and unique dwellers. While charisma has an effect on the success of breeding dwellers, low charisma does not disqualify, either. Even low charisma is reported to work quite quickly. Just keep in mind that the child will count as another dweller, and might grow up to be just another low-level undesirable.
Guards
You could put two of your undesirables in the vault entrance room. Placing dwellers here will put them on guard duty. If the vault is attacked, they will be the first to defend. While you ideally want some higher level guards to defend a more productive vault, low levels make good cannon fodder, and placing high-level guards in the entrance room can be counter-productive. On the rare instance that you have an incident in a room with important dwellers, you could also send your guards in to handle the incident, where you would otherwise be risking the important dwellers. This is likely to be a rare occasion, as incidents can often be more of an annoyance then a risk.
Training
There are many rooms you can build to train a dweller. From 26 dwellers on, you start unlocking the training rooms, starting with the Athletic room. Pop your low levels in here, and build them up on a particular stat. When they have reached the ideal level, you can use them to replace your lowest level, in a corresponding production room. The replaced worker can then be sent to the training room, allowing you a good rotation of upgraded dwellers, while retaining full efficiency in the production room. According to the IGN Wiki, it takes approximately 7 hours to level a dweller from 1 to 5 in agility, using the base agility room. Approximately 3 days will give you a perfect 10.
Just to clarify something, maybe for newcomers, dwellers cannot have negative stats! Instead you see -1, -2, -3 because when the room are filled at their maximum capacity, when you bring someone in, someone else is automatically replaced. The -1 stat is the difference between the dweller replaced (the one with the lowest room specific stat) and the dweller you are trying to bring in.
Also, I usually go for using them as explorers... But don't leave them there... they will bring you a lot of loot. Just give them some moderate equipment and 2 or 3 stimpacks and radaways (especially since you might reach maximum of those really quick - at least I did). I have now 6 of them exploring the wasteland and only 2 of them are like really good at exploring (high endurance, high luck and high strength)