OK I'm 2.5 hours in I think and I'm feeling really overwhelmed and frustrated by the level of detail in the game. I have a ton of shit in my inventory and I don't understand what to do with it all. There's just too much.
I don't understand to sell all the stuff in my inventory. I can "trade" with friendly people but I'm apparently literally just giving them the item and not getting paid for it.
Go to the workbench in Sanctuary Hills (or settlement of your choice), press the Transfer button, and Transfer All Junk. This will dump a lot of crap out of your inventory and into the settlement resources, where you can actually do something with it.
Otherwise, you just have a few things to worry about:
1. What weapons you want to carry/use.
2. What armor/apparel you want to wear/carry.
3. What aid items you want to carry.
Ammo doesn't weigh anything, so carry as much of that as you want. Anything you don't need to be carrying, build a container (chest, whatever) in your settlement and just shove it in there. That's what I do.
I don't understand how the power armor works. It constantly loses power just having it on? Which gauge tells me how much power is left? How do I get more power? It took me forever just to figure out how to store it.
You need to obtain fusion cores. They are found in power generators all over the game, as well as in other power armors. It's one of the meters in the lower right. It shows a percentage of 100.
I also don't understand the building forts thing. I managed to build one machine gun turret but that's it. Now they want me to build a bed, and I have materials but it won't let me build it. I don't get it.
Make sure you have the proper materials, and make sure you are trying to build it in a suitable place. You don't have to be standing at the workshop to build--you can build anywhere in your settlement with the workshop panel open. The bed template will turn green when you're placing it in a good spot.
Also, I nearly died a dozen times over fighting the mercs in that factory in ?Lexington?. I feel like I'm doing everything wrong... My weapons seem to do nothing at distance so I'm basically running up to the people and VATS shooting them in the head...
Try different weapon types. Plain old bullet weapons work pretty well on humans--rifles for distance shooting, pistols and revolvers for close combat. Bear in mind that early on, you simply won't be very accurate. Look at the accuracy stats on your weapons. That will help a lot.
As a tip, if you like laser rifles, join the Brotherhood of Steel and complete the first mission for them. You're given a pretty good laser rifle for your trouble, and it's great for clearing out rooms with fast-moving enemies (e.g. bands of feral ghouls).
You definitely need to be more cautious in the early game. You aren't a superhero (yet). Pick your battles carefully, and make use of cover. This game doesn't have a cover system, per se, but you definitely need to use cover to survive major firefights.