How can I attack another colony ? I have 51 infantry and am a member of a top40 alliance

To attack a colony, you need to have squads

Select the squad you wish to use in attacking (from the map or from the orders menu), and click on the target colony you wish to attack

On the bottom right hand side, on the screen that popped up you have the option to "delay" squads where you can make squads land at certain ticks you want

Check it out on the BattleDawn Wiki:
Squads
Orders Menu

make sure that:

1. u're not conquered
2. the one u're going to attack is not conquered

or the one u're going to attack has crystals

and make sure that u can win if u attack him, do a military scan before u attack.

You don't have to attack someone just for conquering them or their crystals

There's multiple reasons to attack someone

seeing him as a new player I think the only reasons he wants to attack others are

1. conquer
2. killing his conqueror

while 2 is 99% lose.