With most jars you can either heat them or leave them cold. We used to always heat them up, but if it was a hot day etc or a pudding.. we'd give them cold. If you're out, you can leave them cold.. for convenience. If you feed them from a jar whilst you're out or at home, don't use the rest of the contents later.. it needs to be thrown away.
You need to put the jars in the fridge only after you open them and discard within 24/48 hours (it says on the jar for each brand, Hipp is usually 24, Cow and Gate 48 ).
As long as you sterilised or washed the equipment thoroughly, you won't have a problem.. try not to worry about that too much. You can get a Avent chill bag for food/milk if you need one.. apart from that, you don't need to chill the jars as they're not needing to be refrigerated until after you've opened them.
Hope that helped, if you need anymore advice.. just ask.
