I must agree with Undustas but I will develop some points:
Carrusels are for items, not champions. Recurve bow & tear are a priority in the first carrusel, followed by giants belt/b.f/long rod, then chain mail and if there is no other option megatron cloack.
The only exception here can be the first carrusel where you definitely want a 2 cost (green) unit, and only if the tier 2 unit has megatron cloak.
You want the recurve bow because it builds the best item in the game: rapid fire canon. As well as some other powerful items. You want the tear because it builds into statick shiv and shjin. BF for guardian and angel and shjin and giants bell/larg erod for morello.
You want the cost 2 unit because early on getting 2 star units is very important, and it is usually better to sell that unit for 2 gold and buy 2 1 cost units.
Early you usually want to get pairs of the same unit so you rise the chance of getting 2 star.
Rembember, early game is not about winning but about not lossing to much HP, there for do not roll.
It's also a bad idea to hold many units on the bench, and more if this units are tier 3 units. I know its tempting to have two volibears on the bench at level 3, but the chances of getting 2 star is very slim, and if by selling them you can get to 10 or 20 gold you are getting more from it.
Also, dont use your money one leveling up untill you can effectively do it. What i mean is, do not waste 20 gold for getting 20 exp if you still need 10 more to level up. Keep the gold and colect the interset.
while doing this, keep an eye on how many round are left for next creep round because that is a free win that will give you interest, so do not roll down all your gold before a creep round.
Asides, do not comit to items too early, its better to know what item do you want and in what unit before building a bloodthirster because it was the only option. If you get a good item (IE. Morelo), you can give it to a unit you know you are going to sell. Imagine you have a 2 star garen but the rest of your comp are brawlers. You can give it to garen untill you replace it for other unit.
Do not go too hard into 6 units comps, like nobles or sorcerers, they can be really good but they need tier 5 units and this wont appear until later, so its better to have 4 sorcerers and a couple of tanks to 5 sorcerers and a braum.
Now ill give you some personal opinion abotu some units/synergies:
-Phantom is overrated. If phantom is active it is probabbly because you have kindred and morde. Well, morde is not a good unit and kindred should always be paired with another ranger. Instead of keeping this phantom, gettin a CC unit like cho or sejuani can be much better.
-Cho-gath: give this guy morelo and guardian angel. Its better a chogath than a garen even if garen is providing knigth 2.
-if you have a messy game and messed everything up and find a draven, use it.
-Sjhin is good on either ashe, pyke or kazix.
-If you have a spare slot kenen or morgana can carry your team as solo units.
-Volibear is not a priority pick unless you are planing to give him items. (Do not give him guardian angel, if he dies it will revive with 0 mana).
-Braum + thornmail + dragon claw can win you many fights if positioned correctly.
-Wild is not a good trait to go for. Its nice if its active, but do not focus on getting 4 wild with no other sinergy.
-Akali is OP, if you have a spare slot you can use it while you find the correct unit for your team.
I hope this helps.