Hi Alison;
The answer to your question will depend greatly upon the environment in which you will be diving and to some degree on your own tollerance to cold.
If you are divingin tropical waters then 1.5 to 3mil suits are great. However, if you intend on diving on colder Northern waters a 5mil or heavier (even a dry suit) might be necessary.
If you want a little versatility then perhaps a Farmer John/Jane (2 piece) suit might work well for you.
I dive a lot in Central and Eastern Pennsylvania and I use a dry suit or a Farmer John which gives me 3 to 7mil coverage depending on how much of the suit I wear. But when I do dive in Florida (which happens several times each year) I will wear a 3mm suit or shortie...