Question about transferring

The website was pretty unclear so I was hoping to find a little clarity here. I did not get into dartmouth, however it still remains my top choice, thus I would like to try and transfer there. I’m currently a senior in highschool, but I was wondering how soon I can apply as a transfer to dartmouth. Do I have to complete a semester of college wherever I go before I can apply as a transfer? Or is it purely based off of the number of credits I need to attain? Thanks!

For instance, could I send in my transfer application sometime this august when the new common app comes out? This would be before I’ve even started at whatever other school I’m going to. If not, how long do I have to wait?