Yea I've been playing a few games on TTS with my buddy and I get rolled everytime. was trying a few different factions to see what I liked. I think I made the mistake of starting with Dark Eldar. I like the models but I have a hard time getting my raiders close enough to drop Incubi and charge in. havent really looked into their new codex but I really like the Kabalites and Incubi. Shame they lost like a 3rd of their roster w/o replacements.
TTS is a great way to play around with different armies to see what you like. Dark Eldar, even craftworld Eldar, are very squishy and rely on their mobility and positioning to gain advantage. If you want an easy to play faction I might recommend Custodes. It's an elite army so very few units, and each one is very powerful. You mostly walk up the board and kill whatever you come in contact with. The issue with Custodes, mainly, is the game is not friendly to low model count armies since you will often need to skip a unit's turn to perform an action for secondaries. You can bring Sisters of Silence, but they are basic humans who aren't too fast. For the sake of learning how to play it might not be too bad.
I would also recommend Necrons. Necrons are very forgiving in my opinion. They naturally heal themselves every round, even bringing back dead models. They have a good mix of powerful shooting and melee. Plus they have some deceptively tough units. Start simple and run Awakend Dynasty. +1 to hit if a unit has a character leading it. Simple, and universally good.
I think the thing I have the hardest time understanding and grasping is line of sight
You should just learn to get comfortable with line of sight. You may honestly just be over thinking it because it is pretty simple. Imagine you have a piece of string. Try to touch one end of the string to literally any part of your model or base, and touch the other end to literally any part of the target. If it's a straight line you have line of sight. This line can go through, legs, under vehicle treads, as long as you can make a straight line from A to B to can see. In real world scenarios people are rarely nitpicky about line of sight. Even if there's a clump of space marines in front of you people will generally assume you can see over a shoulder, between their legs or whatever to see the target. That is unless you are clearly being hidden by a Land Raider or behind a building. You will have to learn how ruins work because nearly all terrain are ruins, and they have some awkward line of sight rules. Check the Rules Commentary on Ruins (and Visibility) for clear pictures on how it works. I'm assuming you're already using Wahapedia already, but in case you're not, I find their website to be much easier to use even if the PDFs are provided for free.
Basically with ruins, the line you draw for line of sight can never cross through a ruin from one end to the other, even if there are windows or destroyed walls that you can literally see through. Everyone can try to draw lines to anything inside a ruin, but not through it. Anything inside the ruin you would draw line of sight as normal, so you would still need to be able to see what you're shooting through a window or broken section. You are allowed to shoot from within a ruin to targets outside, only if your model is wholly within the ruin. That means nothing is sticking outside the ruin at all. It sounds kind of confusing at first, but I think once it clicks you wont even be thinking about it.
