Build relationships: One of the things that helped me immensely throughout my first year teaching was that I had people who supported me, loved me, laughed with me, kept me on track, and saved my life a couple of times when I truly needed it! Teaching is a tough profession that will test you daily but things are so much easier when you open yourself up to the people in your building.
Collaborate: Teaching is a profession that truly lends itself to collaboration. This goes along with my first tip of building relationships - find people who will show you the ropes when it comes to building expectations, curriculum, etc. And don't forget to share the good ideas that you come up with on your own!
Have high expectations: Do not make the mistake of being that new teacher who has an out-of-control class because you were afraid to be too strict. Set high expectations and your class will rise to meet them. Kids need structure and expectations - don't feel bad for setting them and sticking to them!
Doubt yourself: Teachers have enough people who unfortunately doubt them already. Do not be one of them!
Take on too much: While it is important to be involved and show initiative, do not try to do too much that first year! Being a first year teacher is practically eighty jobs within itself, so focus on that and the rest will come later.
Trust your gut! I have always believed that good teachers have this teacherly-instinct they were born with. Own it!
Now enjoy your last summer before throwing yourself into the best profession the world has to offer :)