1. Choose boots depending on enemy team.
2. Choose Death´s Dance or Wit´s End for armor or magic resist depending on enemy team.
3. Build the item of the 2 you have not build already.
4. Buy Hurricane.
5. Decide between Bloodthirster (more healing + a bit of a shield), Guardian Angel (Generally good and is very usefull against assassins), Infinity Edge (For more damage).

What about Hullbreaker? Why is this not a must buy item?
Hullbreaker is great for Perma-Splitpushing, that´s right. You are able to take towers down faster and you get
resistances if you are isolated from your teammates and you buff your minions. So why don´t we build it?

The problem is, that you can´t avoid teamfight´s forever and sometimes you might fight a 2vs2 etc.
In these cases Hullbreaker is really bad. So unless you think you have no chance of winning a teamfight anyway i would not recommand building it. Buf if you want to build it regardless, build it as your second item. I tried it out a lot and I feel way more comfortable with Death Dance or Wit´s End as my second item.

When you have blue+ white gun you can q the wave with your blue gun ang get a lot of shokerams. This is surprising and you can bait enemys in thinking they will win and than just beat them up.

In generell our weapon order is: Red white green purple blue red and repeat. Most important is that red is in between the red and the green gun the rest doesn´t really matter for Toppelios. We always want red white.

In order to get red white again faster use your abilities just to burn amo and get red white again faster. Literally try to burn through your amo as fast as possible to get to red white again. (Don´t do this in laning phase because you do not have enough mana to do so).

Your combo for being an unbeatable 1vs1 meele champ by the way is: red gun Q into switch to white gun R+ Q and then auto them in meele range. You outdamage everyone and you have the time to do so with your barrier.

If you still have not learned aphelios abilities and what he does in the first place i recommand you checking out this guide from remmacs https://www.youtube.com/watch?v=-exjqh6Md9U
It´s more an adc guide but it explaince you Aphelios kit in great detail.
Still you will probably have to play bunch of games with Aphelios till you are comfortable with him but i can tell you it´s worth it. Aphelios is a really fun champ whith huge potential and once mastered you can produce 200 years yourself;)