please dont play bots. You're missing out on a lot of the game if you are afraid that you will get stomped, dont be, Riot is good at putting you up against people of equal skill. you may run into a few smurfs here and there but those will be filtered out quickly.
as for your question.
you change your build depending on how the game goes. If you get a few early kills, go ahead and build damage items (Black cleaver, Blood thirster, Last Whisper, etc). Brutalizer is usually always a good pick up at the start.
if you dont get too many kills, Jarvan will fall off but if you build tank, you can still be useful to your team. Get items like Warmog's, Sunfire, Randuins. and the like.
Black Cleaver is something i would build almost everytime, but other than that, build for the situation at hand, the same can be said for most champions.
Sig won't change until my sister thinks re4 is a good game. number of people who support=9
get early damage and lifesteal (meaning bloodthirster). all-in every time some puny enemy tries to come into your lane ghostblade --> black cleaver since J4 is all about that ridiculous upfront burst damage sword of the divine helps kill (unwilling) challengers in your thunderdome get LW if you need armor pen. get a second brut after your ghostblade instead if you want to demolish them before they have a chance to build armor (only if fed and the fed enemy is squishy)