Every game I have played with this I demolish the opposing team if you play jg then the more monsters you kill the higher your max health gets even without your ult and while laning your max health will passively go up. During the early game i wouldn't suggest any risky moves because you aren't tanky yet but you can still pull some decent damage as long as you use your q to hit the enemy champion. By the time you get your warmog's finished you can already use it's effects because of your runes so just standing around you can gain maximum health.
However if you chose to only build health then you might not be much of a tank so if you can I would suggest getting a lot of armor and magic resist otherwise you're just a wall of flesh. So even if you have a lot of health people can burst through that like its nothing so I would suggest that if you know someone can burst you down try to counter their damage type (And for people who don't know to counter true damage you could either increase your hp regen or your maximum health).

Always use the bushes to your advantage lure enemies into the bush and use everything you have to burst their hp down low enough so you can ult or just slaughter them.

You can pick any spells but make sure you have flash because Cho'gath can't escape very well.