I don't play Cho'Gath, so I can't really comment on what his strengths and weaknesses are, but I will give some advice to help fix some mistakes that you talked about in the game you played.
First off, try to avoid putting yourself in a lane with three people. Experience gets split evenly between all the people in a given lane, meaning that you get less experience if you are in a lane with three people than in a lane with two people. At lower levels, the top and bottom lanes are the lanes that have two players in the lane while only one person goes in the middle lane, and I recommend you just fill into whatever lane is available such that the side lanes have two people in each. This way, you get a better feel of the mechanics of your champion of choice yourself because you have a few more levels on you.
Something else to note is your build. You said you stacked armor, but unfortunately the enemy team comp had a lot of magic damage between Lux and Teemo. Cho'Gath is a tank, and there are two keys to building tanks.
The first is to build a combination of health and resistances instead of just stacking resistances. There's a lot of math to this strategy, but basically the idea is that your resistances get more value with more health and health gets more value with your resistances, which offers the maximum efficiency for all your defensive stats on your champion.
The other is to build the resistance that counters your opponent's damage. You built armor in your game, which doesn't do much against the magic damage of Lux and Teemo. If you build a lot of magic resistance against those heroes the game would have been a lot more in your favor because magic resistance items will reduce the magic damage you take.
This begs the question: how do I know which stat to build? The simplest way is to look at the items your opponents are building. If your opponents are building a lot of AD items, then it makes a lot of sense to build armor because AD generally signals physical damage. If your opponents are building a lot of AP items, then it makes sense to build a lot of magic resistance because AP generally signals magic damage. Most team comps will have a mix of both damage types, so it's generally a good strategy to build a mix of both, but do your best to build the right defensive stat.