I just started play yesterday and have been wonering how to conquer other castle's. there are other ones around me that are lower rangs than me too.

To conquer a colony you must be not be conquered yourself and you must defeat them in battle.

Don't forget that you have to defeat them at there colony.

Figured that was self explanatory but it is better to be explicit.

if you are conquired, you can still conquire your Conquirer, but you will have to defeat him/them, on your Conquirer Castle/colony, and after doing this, you will be free

Gonna get of topic..so a question..if you are conquered..can you steal crystals??

Oh yea, you just can't conquire any 1, other than your conquirer... you can do any thing else you want... like Taking crystalswreckage taking, taking OPs, Taking Relics etc
edit: Don't forget to apply for a mentor here http://www.battledawn.com/forum/viewtopic.php?f=94&t=1036

Conquering a colony occurs whenever a battle at a colony results in the owner of a colony losing but only if the colony was originally not conquered by anyone, and the victor was currently unconquered.
When you conquer a colony your squads return back to where they were sent from (e.g. your colony). Colonies can not be conquered if they have already been conquered by you or someone else or you are conquered yourself. The only exception to this is when you attack your conqueror, you can conquer your conqueror ONLY when you are already conquered, and doing so will liberate any conquered colonies by your conqueror.
Sending squads to attack colonies that have already been conquered is a waste of time. Doing so is often seen as 'griefing' and may result in.


TIP: Conquering your conqueror;
If you are conquered, and your conqueror is close by, and someone in your conqueror's alliance is beside your conqueror, send an infantry unit (The most basic infantry unit.) towards each of their outposts with no units in it, conquering their outposts, (it's all right if they re-conquer their own outposts, that's the point of this trick) and this will draw away (most likely) every unit they have AWAY from their colonies.
Then, attack the other guy with that 1 infantry unit at the SAME time as you attack your conqueror all-out with as much units as YOU can make. (Make sure to jam BOTH of their radars first!) As this is happening, the SAME tick you sent both of the squads out, (send them out on the same tick, so that they arrive, more or less, at the same time) relocate your colony a long way away from your conqueror. This will occupy your conqueror (Soon to be conquered by you.) while you are being liberated by the relocation.
Also, they wont know where you are, and in the 3+ ticks it takes to conquer your conqueror, while you have relocated and are still rebelling, you will be automatically freed by the conquering of your conqueror.


Source
Taken from battle dawn wiki.