Judging by questions on this site - which I admit are probably not representative of beginners as a whole but which are, at the very least, of special interest to readers likely to peruse this answer - I would say that one basic mistake, which leads to a number of others, is not stepping back and getting a general sense of the capabilities of TeX & friends. I don't mean the details, but the 'big picture'. For example,
- TeX & friends are good tools for typesetting text, mathematics and bibliographies;
- TeX & friends are great for automatically ensuring consistent layout, accurate references and cross-references;
- TeX & friends are not the best tools to create a database, manage a spreadsheet or draw elaborate 3D scenes with accurate perspective and lighting;
- TeX & friends are not good replacements for tools such as
grep, although many editors provide much of their functionality;
- neither TeX nor friends are good replacements for a sheepdog;
- TeX & friends make at best indifferent tea.
In particular, TeX & friends are much better than human beings - and much better than many other programmes - at the things they do well. They are not generally good - and typically much poorer than alternative programmes - at the things they are not intended to do.
One key strategy for getting the most out of TeX & friends is to use the right tool for the right job. Some questions boil down to requests for TeX solutions to the problem of brewing the perfect cuppa. This is a perfectly reasonable aim but the best answer involves advising a non-TeX solution.
Other questions essentially ask for TeX-based cat-herding solutions. The best answer in this case involves recommending against adopting the goal set out in the question.
However, these types of answers are rarely appreciated and frequently interpreted as either lacking in imagination ('I don't believe it is not possible/advisable') or as admissions of TeX's failure to rise to the needs of twenty-first century users ('Super Programme can make tea while simultaneously sheering sheep, sending a letter to my gran and paying the gas bill'). This is a shame since these really are the best answers in these cases and appreciating TeX's limitations, as well as its strengths, is key to using it effectively.
[That is, I am extremely sceptical of solutions of this kind.]
Cup of tea is from openclipart.org.
'Like Juggling While Herding Cats' is by Robin Catesby and available here under this CC licence.