Hi Rob,
First of all, welcome to the community!
Taking over someones base is only possible (and allowed) if you forget you maintain your base and your plot pole disappears. No plot = no owner, so everyone is free to put one down.
About base building:
To build a base, you'll need to find a spot on the map. When you find one press F3 -> Get starter kit. You'll notice it'll often not let you place it because you'll be too close (400m) to a other plot pole/base. I suggest to find a location on the west and/or north side of the map, because the mid and eastern sides are crowded and finding a spot there is tough (but not impossible!). So when it allows you to put a starter kit down, place it and put the plot pole down which is also in the kit. The starter kit has more building materials and some money to help you start out with a small base on that location you've just chosen.
About the materials for the base.
I suggest you later upgrade your base with either cinder or metal floors. They're the most versatile and safe from gunfire. But none (nor wood) can be destroyed. Only the doors can be blown off if you put a lock on it. I also suggest to go around the map and see other bases to get some inspiration.
Hope this helped, if you have any more questions, send them here, in-game or on the server Discord.
Have fun and good luck! ^^