I just got my kit and am about to brew for the first time.
I was warned to not use bottles that have sat out uncleaned because there can be mold in them.
How can I tell if there is mold in my bottles? Can I safely clean and remove the mold?
It can be hard to see a thin ring of black mold in an amber bottle, even when holding it up against the light.
I'd strongly recommend you use a bottle washer on a faucet. Really hot water doesn't kill everything but it does tend to clean well. Here's one as an example: http://morebeer.com/view_product/15964//Bottle_and_Carboy_Washer
I'd also recommend you get a bottle tree or two so you can clean bottles at your convenience and have them waiting and ready when you need them.
FWIW, here is my ritual:
Are the bottles of unknown cleanliness (eg from a a party, donated by a friend, etc)? If yes, then they get a bottle washing using the device I linked above and go into a sink/basin/plastic tub with sani solution. As someone else mentioned, PBW might be a better for this step as it would likely break up soil/dirt/mold better than star sani. If you use PBW be aware that you need to rinse much much more thoroughly than with star sani. The bottle washer works well for that.
After the first round of cleaning or if they were my bottles that I know were rinsed clean after use, I use the bottle washer on the faucet on them again and then use this device to inject sani into them. Here is an example device. Mine is a little different but same idea: http://morebeer.com/view_product/15682/102303/Sanitizer_Injector If you don't have this device, you can just soak them in sani.
Then they go onto the tree to drain the sani-water out of them and await use. I'm sure you know what a bottle tree is but if not, here is an example. http://morebeer.com/view_product/15677
Personally, if a 12 oz bottle is so dirty that the faucet cleaner and a soak doesn't get it looking good, I recycle it. Bombers get the brush.
If your bottles are truly filthy (dumpster dumping etc), I would consider soaking them in a bathtub or garbage can in bleach for a few days before cleaning. Be aware that bleach is very very bad for your beer so if you use this pre-step, you'll need to rinse really thoroughly. My brother used to use this approach in college on dumpster bottles.
This website has some good info.
I put a large plastic tub in the bathtub and fill it with hot tap water mixed with the oxygen cleaner from the dollar tree (yes, it has fragrance, but it's cheap and rinses clean). Splurge and use $0.50 worth! Into that, I immerse bottles, and let them sit over night. The next day, I use a home-made bottle rinser to quadruple rinse (spray the interior, let drain, and repeat). The bottle rinser is a chunk of cpvc with tiny holes drilled in it. The rinse water is as hot as I can get. Usually I start with rinsed bottles, but even some trashy one come clean with the soak and rinse.
I wouldn't even START talking about sanitzer if I could see the tiniest spec of anything! Only after my soak and rinse, then I hit them with sanitizer. They all go back in the case, I lay clean paper towels on the top, close the case, flip the case over, and store until needed.
You will most commonly see the growth of mold on the bottom of the bottle, when holding the bottle up to a light. If you do see mold, fill the bottle with your sanitizing solution and scrub with a bottle brush. Then, if it was me, I would rinse and sanitize once more
If after sanitizing your bottles you can not see any mold in the bottle, you are OK.