In all honesty with you - whether you do a tour or not, you will always have to travel on the plane alone and get to your first hostel/hotel on your own. You do not all leave together and meet at the airport and travel together from your home country. You will have to make your own way usually to your first accommodation and only from there do you travel as a group.
I do understand your nervousnass as I was the same when I first started travelling and I still think that doing a tour is not a terribly bad way to start it off if you have never been away on your own before.
However, saying all that Thailand is definately one place where you do not need to do a tour as everything is so easy and very set up for travellers.
Yes it will be very daunting at first but you will soon get into it.
Tour companies are very expensive ways of seeing a country but I do think they serve a purpose especially if you are very short on time - they organise everything and get you from A to B in a convenient way although it can very much be - get off bus, take photo, get back on bus. You will still get to see the country but its very very different to travelling on your own independantly.
The best example I can give you is my tour I did in India - the first place I visited on my travels where it was just one temple after another temple after another temple. Had I done my travels independantly I would have done it very differently however I didn't have the nerve to tackle that country on my own.
As you've said, you've been surfing the net looking for advice and you will see from what other people have posted about being nervous going away on their own and all the replies they get back about it being the best thing that they have ever done.
If you do go on a tour, i'm sure you will enjoy it, but i'm even more sure that if you go it alone you will enjoy it even more. On a tour you are stuck with the same group of people for 2 or 3 weeks - that can be a very long time if you don't get on. Of course, you may find a great group of people that you really hit it off with.
Read more posts on Thorn Tree about other peoples experiences and get yourself a guide book and read it thoroughly - it will give you a great idea about how to get around Thailand and how easy it really is even on your own.