I am about to develop a project in yii2; but I fear that the changes in yii3 are very hard, I do not want to invest a lot of time in migrating the code. On the other hand if I wait for the first stable version of yii3 I would start from scratch and from here to an indeterminate period.
What dou you recommend?
I wait, or start writing code?
If I start with yii2, what aspects should I take into account so that my migration is as easy as possible?
Nobody here can know better than you how long you can afford to wait. And nobody knows when 3 will be production ready. You have to have to make this call yourself, unless you want me to toss a coin for you 
But there is a 3rd option that also works. Write the app in Yii 2 and keep it that way – don’t migrate. Yii 2 isn’t going to stop working or go away. It’s going to be ok, really.
Since Yii3 is still on the skies, start with Yii2. Though many things will change, You can migrate slowly. I don’t see why you can’t start with Yii2!
Start with Yii2. I still maintain a couple of Yii1 applications, and within the known capabilities and environment, they still do their jobs nicely!
Dont wait. Yii2 is Good to start till 3 comes.