First things first, I'll talk you through my (somewhat naïve) beginnings in microstock. I first learned about the industry while looking for ways to make money online as a slightly poor Sixth Former (I decided that sitting around taking surveys about gravy adverts wasn't going to be a compelling way of creating some extra income...). I had been taking photographs at a casual level for a few years, so had some images to work with, so this was where I decided to start and I strongly recommend you do too - I admit, that probably isn't the most inspired piece of advice, but I did learn a few things to look for that would have saved me a lot of time while looking through:
1) Check that your photos are technically perfect.
I didn't realise how important this was when I first started and hence wasted a lot of time during my initial applications. It's been said a lot before, but reviewers will check every pixel at 100% size, so make sure you do the same - don't fool yourself that it's perfect when it's not (guilty!), if the photo's even a little bit noisy, they're going to notice and there's nothing worse than waiting 10 days for a photo to be reviewed and then to be rejected because you didn't spend a couple of minutes looking over it properly
As well as this, make sure the photo is well lit and composed. This was my main reason for rejection when I started and, despite it often being a little ambiguous what's wrong, you can easily pick up the type of photos that get accepted by doing a quick search for your subject. Some would class this as copying other contributors and their styles, but you'll quickly learn that a lot of photographs are very similar in style and composition and, while creativity is important, using a successful style of lighting and composition may be a good way to start.
2) Your photos need to be good stock photos, not just good photos.
When I first started, I had a fairly large portfolio of images, some of which were quite good and I fell into the trap of thinking this would mean that I'd be making thousands in my first few months. As you may have guessed, that was not the case... It soon became apparent that people by photos of things they need, not photos that they think look nice. That said, I had no problem getting them accepted to any of the agencies - they just didn't turn out to be the kind of image that sells particularly often; I have had actually had a few sales of the 'weird' photos from my applications (namely a blurry close-up of a rug and a pelican crossing "WAIT" sign), but nothing to write home about. I have no idea who bought them, but boy was I glad when they did.
So feel free to use your technically accurate images of less popular themes in your applications, just make sure you hold off from buying your next lens for a little longer.
3) Don't bother submitting any photos of property that you don't own,
These pictures will end up in the Editorial section on most sites and, unless the row of shops you saw on holiday comes into the news, you're probably not going to sell many of these photos. This includes obvious things like buildings, shop fronts and logos, but also less obvious things like boats in a port. I got caught out by this when I first applied to 123rf. I took a few photos of Cassis, France on my summer holiday, so I decided to submit one of the waterfront and one of some small boats by the port side. The photos were accepted to the Editorial section, which I was pleased about at first, but have remained untouched ever since - I'm still waiting for a breaking news story about those boats...
There'll be other bits of advice on millions of other blogs, but these are the things I wish I'd been told before I started. Have a look through your old folders of images again before you start applying, chances are you'll have a couple of gems hidden in there somewhere - just make sure that you double check their quality and that they are 'generic' enough to be used as stock. And don't be disheartened if your files get rejected the first couple of times, they don't know what they're missing! Or so I told myself...