I disagree, Popa. Best to write an abstract or an outline. Once you have done this, you can just fill in the sections. It really helps you plan out the whole thing. If you just start writing randomly, you will ramble on. Deciding what points you want to make, in advance, and how you will prove them and put them together is most of the battle. Then, actually doing it is much easier.
I went through all of this in writing a thesis.
Remember, a formal outline is something like this:
I. Intro
II. Chapter 1
A. First section
1. First subsection
2. Second subsection
…
B. Second section
…
III. Chapter 2
…
…
IX. Summary and Conclusion
X. Bibliograpy and Endnotes
Note you can even have III, A, 2, a, iv, etc., but almost nobody uses that level of detail.