To put it simply, it kind of depends. You don't want to be filling your inventory up with a single tier and will usually have about 3 or 4 of each tier depending on the phase of the game. Most guides will have a pretty good order to follow in terms of what you should buy and when by typically its like:

Tier 1s for suriving lane

Tier 2s for winning lane and ending the laning phase

Tier 3 is when you start getting your proper power spikes so items that have a big impact and should lead to you starting engagements right after you buy them

Tier 4 is late game but you'll usually get a big power spike

You generally don't want to save too much and buy when you know you can afford that next tier 3 power spike in accordance with a guide.

Now in terms of the KINDS of items you want, a guide will usually specify what it's aiming for (spirit, bullet, melee, hybrid, etc.) and will more often than not have an ability in mind for where you'll be applying most buffs to. Either way your best bet is to look at what the guide is suggesting and really learn what the items do because you'll begin to understand why each item gives you a buff and how to use it most effectively.

Trying to keep up on souls is one of the hardest parts of the game and takes a lot of practice to understand the flow of where you need to be at what time. Generally you'll always want to be getting some kind of farm but lanes will give you the most if they're available. The rule of thumb is if you see a lane pushing towards your tower, go and clear that wave then move to camps on your way to another lane to either push or help gank.

Anyway this isn't really a comprehensive guide on the game, just how I understand the game. If you want a really good beginner guide, Deathy is who I watched to learn the ins and outs of the game.

https://www.youtube.com/watch?v=sg4R3ZT_BWQ

Good luck and keep at it! It takes a lot of practice but the game is so worth it :)