Everyone has different reasons for deciding to make the switch. My first bit of advice it to be sure you know why you want to "go green". For me personally it was knowing that the skin is the largest organ and whatever you put on it can get absorbed into your bloodstream. I basically had a sudden moment of realisation: 'Why not let good things into my body rather than toxic chemicals?'. Aside from that I have the odd outbreak of eczema on my hands and also have relatively sensitive skin so I wanted to use products that would be a bit more gentle.
Once you've figured out your reasons, a really important part is to clue yourself up on ingredients. A good resource is EWG's Skin Deep Cosmetics Database which is essentially a library of ingredients and products which gives them a hazard score from 1-10 (low hazard-high hazard). Sometimes on the Skin Deep website an ingredient might be given a higher hazard rating (or sometimes even a lower rating) because there's isn't enough data on it, so watch out for that! You can also do a web search on toxic cosmetic ingredients and you'll find an awful lot of information. It can all be a bit overwhelming and there's a certain amount of scaremongering out there, but decide for yourself what specific ingredients you want to avoid and how "clean" you want to go.
In terms of how "clean" you want you want to be, give yourself an overall percentage rule - I personally have an 80/20 rule for buying new products, so the 80% majority are natural, but I still give a 20% leeway for the odd few products I can't replace yet (or can't quite bring myself to give up yet!). I know some other green bloggers have a similar sort of rule, especially for things like perfume or the odd item of make-up.
You don't necessarily have to go cold turkey and throw out everything (unless you are made of money!). If you do decide to get rid of everything and start from scratch though try to give some of your old products to family or friends who will make use of them. Switching to natural products can be very expensive so honestly don't feel you have to switch everything at once. At least start off with products you use most often and look in to getting a better alternative when you run out. I've been gradually transitioning over the past 6 months or so and now all of my skincare and bath/shower products are natural. My make-up collection on the other hand is still about 50/50. Just remember it's perfectly fine to do it gradually, whether it's just as and when you can afford it or when you finish off an old product.
Yes you might make the occasional mistake at first and buy something thinking it's a good one, only later to find out that one or two of the ingredients are on your avoid list. It's happened to me before, but you can learn from it so try not to worry! Also if you don't know what's in it, don't buy it! You'll start to notice you spend a lot of time reading ingredients lists when you're deciding if you should buy something!
Some of my favourite places to shop online for natural beauty products are LoveLula, Naturisimo, abeautifulworld, Lucy Rose and Content Beauty. You can also find an increasing number of more natural products on mainstream sites like Feel Unique now too.
It can all be a bit difficult for a while, but it really does get easier with time and you skin will definitely thank you for it! Also if your family or friends don't share your opinions and think you're crazy, so what?! It's your decision, don't let them make you doubt it :) If you're struggling to find like-minded people there are a lot of lovely friendly green bloggers out there and I've found chatting to them on their blogs and through twitter a great help. There's also a weekly twitter #greenchat run by Ana (from Ana Goes Green) every Thursday at 8pm UK time if you're interested in getting involved. (Update: the chat seems to be no longer active).
Hopefully this post wasn't too long and at least someone has found this helpful! If you've already "gone green" feel free to share any tips you might have in the comments :)