It's fine to start without defense plans. The first pressure is to get some energy into your controllers within about 24 hours, which is fairly simple.
When you can, build a tower, give it energy, and write code to have it attack things. Towers are extremely strong defense.
In terms of starting locations, the farther away from other players the better. If you change the map to show you "energy mined in last hour", pick a spot where nobody is mining, and ideally where nobody is mining even adjacent to that. It'll give you some good buffer.
Alternatively, look for rooms with a highlighted border - those are newbie spawns, and being next to them is reasonably safe in comparison to someone with high level murder bots.