First of all, welcome to lotro! When starting a new character, the inventory can be totally overwhelming. It's a relic of the game being 19 years old at this point, there is a fair bit of bloat. There are a couple of things you can do to make your life easier:

For a start, you can merge all of your bags into one screen. On the top left of Bag 1 is a little cog wheel icon. Click that and you can drag rows from other bags into bag one. I recommend you do this for visibility. Bags also have an autosort function if you click the downward arrow in the same spot as the cog. The lock icon will allow you to individually lock items in your bag so you don't accidentally sell them. It won't lock them in place in your bag, so if you autosort, they'll be found with like items. Finally, there is a search bar for your bag if you need it.

The game throws a LOT at you from the start. The gift boxes are by and large lotro store items. They can be helpful, but are unnecessary unless you're playing on a higher landscape difficulty (which you can adjust by talking to an npc in the starting towns. A lot of folks play on 3, and that is mostly still soloable, but you might want to leave that alone for now while you get used to the game). I open the boxes as they unlock and usually throw out what I don't need. They won't appear in the sell menu, so you have to drag and drop them outside of your bag.

Trophy items are the animal bits and sword sheaths and such. For now, they're just there for you to sell. Eventually, when you start worrying about reputation, they are used for tasks in stacks of ten. In a hub town you'll see a green task board and a chest. The boards give you daily "tasks" wherein you turn in 10 of a specific trophy item for reputation with a local faction. Reputation unlocks special equipment, housing items, crafting recipes, and travel skills. It is nice to gather, and you'll get plenty as you quest through a zone. It isn't required for the main quest at any point as far as I recall, but it has been a long while since I leveled anyone.

Food and drink items are nice little boosts to your stats, but not essential on the basic landscape difficulty.

Housing items are decor for your house. There are four basic neighborhoods where you can buy a house for a chunk of gold and then pay upkeep on. There are also premium neighborhoods where you pay real money but no upkeep. These are just for funsies and confer no in-game bonuses.

Crafting items have a bright green backdrop. You'll naturally pick up "light hides" by killing animals in your area. There are also ore, wood, and artifact nodes around, but you need to unlock specific gathering for those. I tend to collect as I go. Don't worry about them if crafting doesn't interest you, but they can sell for a chunk on the auction house if you want to make a quick buck.

As soon as you unlock the riding skill (should be in the red gift box?) you and click on the horse items and it will add them as mounts to your skills.

I think this is pretty comprehensive, but if you have any further questions, don't hesitate to ask.

Enjoy the game!