First Project (auto garden)
There's a few similar projects already done on YouTube, which you can use for guidance. If this is your 1st project using arduino I would recommend the Uno as is the easiest to work with. As you become more familiar with microcontrollers you can easily switch to the nano.
As for what sensors you decide to use is up to you. You have quite a bit of flexibility. Check out Adafruit and arduino websites for ideas. What you choose to go for would depend on the specifics of your project, which a bit vague in this post.