Most probably we are going to destroy ourselves before any alien can reach us.

But lets say aliens do invade , well here`s how I think its going to go:
- First we wont be able to know about it until it would be too late , because the government wont tell us anything.

- In case of non-friendly aliens , we are to assume we will be quite dead if they wanted to , if they have the technology to fly millions of light years to us , They sure have the power to erase our DNA from the planet.

- In case of friendly aliens , I doubt it will be an invasion , but if there will be one there will be to rescue us from a threat that is to come and they know that we will not survive.