Mostly, just be yourself and wear what is comfortable for you. If you wear something specifically to look more 'attractive' (which is a relative term, really) but aren't comfortable in it, that's going to show and it's going to make you look less attractive than any physical (I'm assuming that you mean overweight, since that is usually what people use the word 'big' for) 'flaw'.
If by big you mean on the heavy side, I suggest something like a bowling shirt--yah know, cut wise. I say this 'cause you wear them untucked which helps with a bigger belly, but they still have structure to them so that they still look pulled together or slightly dressy. Wear black or darker pants since they are slimming & dressier as well.