I would advise you to do some serious research first, not only on the actual content of your doc but on how marketable the subject matter is likely to be (locally, nationally and internationally) and also research your local TV station as well. For example, does you local TV station actually ever commission docs or do they only buy in.
Local TV stations tend to have relatively complex business structures which involve some sort of affiliation with a network. It could be that your local TV station doesn't directly commission docos, only their affiliated network does. The real money in docs is in syndication, selling licences to broadcast to various affiliates and other broadcasters in different regions and territories. Local TV stations may not have the legal expertise/personnel to get directly involved in syndication except in terms of broadcasting content essentially licensed on their behalf by their affiliated network. You will need to research and understand how your local TV station operates in this regard, how your proposed doco fits into their business structure before you approach them. Maybe it doesn't fit at all and you should be approaching their affiliated network instead. If you don't have a decent knowledge of these issues you will most likely come across as a complete beginner with no idea of what you are doing, which of course will drastically reduce your chances of getting any support/investment in your doco.
Unfortunately, when you start looking to make money from documentary filmmaking that means entering the documentary business world and you need to understand how that world operates and what it requires to stand any realistic chance of success. This is where so many inexperienced indie filmmakers fail; they just go out and make their film and when they've finished they pat themselves on the back then try to sell it. But who are they going to sell it to? They have to find a broadcaster who: 1. Is looking to buy a doco at that instant, 2. Whose program schedule has an available slot which the doco fits, 3. Whose audience demographic matches the doco's subject matter and style and 4. Who are willing to pump in the time/money to make the doco compliant with their broadcast QC requirements. In practise, that rules out pretty much EVERY broadcaster and without some miraculous stroke of luck renders the doco virtually worthless commercially!
To a large extent this is true of all indie filmmaking, not just docos. In short, find out who your potential purchasers are (local TV station, network, etc.), find out what sort of product they want (duration, subject matter, target audience), find out what technical requirements they have and a rough idea of their budget for docos and then work out a business plan and proposal which allows you to meet those requirements. If you don't do this you are just shooting in the dark (without knowing if there is even anything within shooting range!).
G