As merciful says, joining a sub is normally a good start and can teach you a lot about the game. To join a sub either put a message in the recruitment section of the forum or just plant on a world (or 2) and see which alliances have subs and which of them are open meaning you can join straight away.
Each of these basically leads into the other. Assuming you have no alliance upon entering a world the best thing you can do is look strong on your own.
This can be done by taking conquers really quickly as for each conquer you gain 1 power and a certain amount of resources a tick. Conquers that are easy to take are the AI colonies that have 0 achievements and 0 units recruited.
You’ll also need to be careful to not use all your resources on just creating armour units. Although they are cheap and easy to mass produce making it look like you have an army they are in fact the weakest unit by themselves. To create stronger units make sure to get the damage and range structure upgrade to allow you to take less losses in battles.
Assuming you get a few conquers (10+) and don’t get conquered yourself hopefully a top alliance will take you in and be able to help you get stronger.
If you want more detail feel free to add me on skype: elcent95
Alternatively @Senatus has created a whole guide to BD that helps people a lot: http://community.tacticsoft.net/t/senatus-long-and-pretty-complete-guide-to-bd/46