The lens I want to save up for!

[Overread]

the only advice I have which is mostly confirmed is not to go for stock agencies - your photos sell for peanuts and you have to have a lot of images and have them sell to start to make any sort of return. Further any image sold via that method really can't be sold on again at a higher profit since its already distributed for cheaper.
From people I have spoken to they have said that its a lot of work (especailly today) to get any sort of return from micro stock

As for what sells - people tend to sell pretty well. Weddings, portrates, celebraties - most other things don't tend to sell as much, landscapes do well (if your good) though wildlife and insects hardly sells at all; if you can get into product photography that is a line and often people do product shots or portates for their employers (though do check this out as it can be a minefield with a few nasty problems that might crop up)

Honestly a lot of people start small - keep the day job, but work at weekends and in freetime with any photography work - its once you start to have to reject photography work (in what ever line) because your working the dayjob that it starts to become possible to consider switching