To start off, I would highly advise that you scout your target before every attack. You never know what may be up their sleeve. Recruit a hero with a high intelligence rating and use him for scouting. You will yield better results. You will also need to upgrade your rally spot to at least level 2.
When attacking a valley, I would suggest doing so with a large enough army that you outnumber the opponent 3 to 1. This will guarantee you very low casualties. Try to only conquer valleys that are NOT controlled by another lord. The reason for this is because conquering land that is someone elses’ could lead to a war between you two. This would only cause resource loss on both sides.
Determine what resources you want to add a production bonus to and find level 1 valleys nearby and attack them. After the attack is over you have to tell the army to return, otherwise they will just camp at the valley. You will need as many soldiers as you can as you expand your conquest.
After you have all the level 1 valleys that your town hall can support, upgrade the town hall, now you can attack another 3 valleys (for each Town Hall upgrade). Keep upgrading the town hall until you have a level 4 town hall. Conquer as many level 1 valleys as the town hall at level 4 will support.
Now recruit some more scouts, preferably more than 30. Scout a level 2 valley. Research archers and recruit some them, depending on what army you will be facing in the level 2 valley you just scouted. To be safe, get at least 50 archers.
Recruit an army so that it is 3 times larger than the army in the level 2 valley.