Talk to me about multivitamins

[NotAMammy]

I decided to try and get my health up to scratch and as we will hopefully be ttc next year (or so) I thought it would be best to start taking a multivitamin with folic acid. I just picked up Boots A-Z to start me off, but then I've read an article saying that multivits are actually bad for you and you should just take what you need. But how do you know what you need?
Is multivit the way to go or mixing and matching best? And is there a difference in brands, or am I as well getting the cheapo own brands (as I do with hayfever tablets, etc)