힘내요 means cheer up. You can tell someone who are depressed or in blue to encourage them to do anything. 힘내라고 means the same but it is more polite way to say that. Specially to people who are older than you. Is it good for you to see the difference. However, as a foreigner you can say either one before you get used to the culture in spoken and written korean

So 힘 means cheer, strength or force and 힘들어 means hard / difficult? And if I'm asking someone if he's feeling tired / hard, can I say 힘들어지???

I am sorry my previous comment is for 힘내요 and 힘내라고 However, your question is about 힘들어요. 힘들어요 means " I am tired or something is very hard" so if you are asking someone with it it means "are you tired?"