Need Help as Beginner Cho'Gath
I've lately picked up Cho'Gath and have so far played 8 games as him. While I actually find him to be pretty easy and fun to use, I have only ended up winning 50% of my games thus far, despite the fact that I win lane in every single game (since Cho'Gath is naturally good in lane); thus, I'm assuming my problems arise mid game in areas such as roaming and itemization. I think I do fairly well in team fights, but in games where my teammates are relatively weak, it's somewhat difficult to keep them safe even with my Q and W. Are there any good team fighting strategies that would improve my Cho'Gath play if people like my adc or apc are lagging behind but can still come back?
Beyond team fighting, I'd also like to ask questions about roaming. Although Cho'Gath is very slow, he does have good cc. I recognize that if I'm doing well, it'd be best for me to share my power with my team and hopefully make life easier for the other lanes by giving them kills or assists. Sadly, I don't know when to roam. I know that when I decide to roam, I should clear the current wave as fast as possible and push it to the enemy tower, then leave to another lane, but I'm not sure when it's appropriate to do so. Are there any clear times when it's NOT a good idea to roam? What if I'm going even with my laner (e.g. we both have 0/0/0, or 1/1/0, and cs remains relatively the same)? Should I roam then? What about when we're going even but my cs is relatively higher than theirs? The reason I ask is simply due to the fact that Cho'Gath takes a very long time to get to other lanes and back to his. I mean, I usually use teleport, but sometimes when I'm top and I want to gank mid lane and there isn't a ward, I have to walk there. Should I get boots of mobility for such situations? It would help with roaming, but it wouldn't provide the same benefits as merc or sorc shoes later on in the game (the latter I actually haven't gotten in a game yet).
Also, concerning itemization, if I'm doing well but overall my team isn't, should I go more tanky or more damage? Thus far, I've been building an initial damage item and then building the according tank items to match the enemy team, and after 2 or 3 tank items, I decide to get another damage item. I usually start ROA on Cho'Gath and because farming is easy with his E ability, I can get it fairly quickly. I'm not sure what other damage items I should get beyond this. Would Trinity Force be a good follow up damage item in the later game? Iceborn Gauntlet? Zhonyas, Rabadon's, Abyssal, and Rylai's Scepter?
If anyone could help answer these questions, it'd be greatly appreciated. Thanks!