first of all I would like to welcome to the game, and if u haven't already, take a look at our wiki site
which has every details about how our game works (buildings, outposts, troops, etc.).
Plus u may wanna visit our Adopt-a-Newbie Center
and apply for a personal mentor, who will guide you through the game to at least intermediate level.
Second, to answer about ur questions, joining an alliance is definitely a must for anyone, regardless new players or pro (unless u're in a solo server). However, you have to make sure that the alliance that u're joining is a good alliance, the members must be active, helpful and friendly.
War between alliances is a must and one of the main purposes of this game. Believe me, if ur alliance is good, going to war is a very good experience for you.
And wars are usually declared on the broadcast, make sure to read all of them to catch up with wat is going on in the era.
And like going to war, attacking and conquering weaker colonies is also a part of the game. It brings you an average of 5 metal (or gold, mineral) and some oil (wood) per tick for each conquer, so everyone in the world will try to get as much conquers as possible while defend themselves from being conquered.
Ur protection will last for 100 ticks, u can see how many protection ticks remain in a small box below ur colony.
And as I stated, people will try to conquer as much as they can, so when ur protection is over, expect an attack soon. There are no way to see exactly how many soldiers they currently have, but u can do a military scan to see the approximate number of units on a specific OP or colony.
However, u can try and guess their unit size with by look at their power count and guess the worker count. More about power, please take a look at the wiki. The power counts are displayed on the player tab on the "Community" window.