(a) slip knot (where it all begins),
(b) how to chain (the foundation of all things crochet), and
(c) single crochet (and there you have it, you know how to crochet), the most important lesson is double crochet, from side to side, and controlling your margins.
Statistically I can't say that the majority of what we crochet contains double crochet, but in my personal experience and that of people I've taught who choose their own projects, double crochet is a major player, even if often combined with single and triple.
When you're working double crochet in the round, it seems easier to keep track of your stitches without counting and paying attention--you go on "auto pilot" sometimes. However, when you're working double crochet from side to side, you shouldn't go on autopilot until you have learned to mind your margins. Pay strict attention to your stitches, especially when you are a beginner. Its very easy to unintentionally: increase a stitch on the right hand side, and decrease a stitch on the left hand side.
Since we increase by making 2 stitches into the same stitch or space, and the initial chain 3 that is needed at the beginning of a DC row takes the place of the first DC, its crucial to pay attention to working the 2nd stitch and not placing a DC into the first stitch/same stitch as where the 3 chains reside. That -- would give you a resulting increase and ... too many stitches.
At the end of the row, NOT working into the top of the 3rd chain on the 3-chain-turning-chain-that-took-the-place-of-the-first-double-crochet-of-the-last-row......WHEW...creates a decrease. If you happily went along crocheting and not paying attention to these rules, it might appear in the first row where this happened that all is well. It might look fine after the next row also. However, at some point those stitches start making a visual difference, and then you have to rip back your work and begin again. Not so big a deal if your stitch count is 20. If your stitch count is 120 and you've made this mistake 5 rows back....ARGH....its enough to make you mad (that you didn't pay attention).
If it looks fan shaped, you've been adding stitches, most likely by working the first stitch after making 3 chains--which is an unintended increase. If your project looks like a triangle, a trapozoid, or some other shrinking margin, you've not been working the turning chain/last stitch of the previous row, or a combination of issues on both ends.
Your first course of action in ensuring "goal post" margins is to utilize the rules of double crochet...that is: 1) 3 chains take the place of the first DC, so make the actual DC in the 2nd stitch; and
2) work that last stitch which is the turning chain.
Another method to employ is to count your stitches after every row, as a backup.
Also, look at your work from time to time - stop, look, see that all is well (or not).