The following image contains 3 examples of common mistakes made at the beginning of games (the opening). All 3 colours shown have made errors (this isn't an example of a real game, it's just 3 bad starts shown on the same board).
First consider the Blue move. This move looks tempting because it's nice and symmetric, and seems to use space ``efficiently", because you're starting right against the wall. But it is very important to get to the middle quickly, this move is far too slow (among other problems). It's just not worth it.
Now look at the Yellow move. This move has similar problems to the Blue V. After realizing the problems with the Blue V, it's tempting to make this choice instead, because it has all the ``nice'' aspects that the V had, but at least gets you a bit closer to the middle. But it could be getting you even closer to the middle in a different orientation. Just don't make this move. It's bad.
Finally look at the two Red moves. This kind of start can also be tempting because it seems to be creating i1 holes very quickly, However, the Red player will quickly run out of space to expand into, and will have already filled the space he/she staked out. Again, early in the game, the key is always expansion. Defense can come later.
Here we see the same pieces played in better orientations -
away from the corners and toward the center.
The biggest thing to take away here - if we're not expanding toward the center at the beginning of the game, we're making a mistake.