I had this same dilemma for my MIL's wedding this fall and made the Crepe dress from Colette Patterns. It is one of her only beginner patterns and I can say that it was very easy to put together (no tricks or difficult steps outside of darts).
The best part is that it's such a simple dress that you can use fabric to adjust the look according to the formality of the event - formal? Silk shantung. Casual, beach wedding? Seersucker or cotton.
Hope this helps - I know finding this pattern was a God-send to me at the time!
Charlotte, keep the pattern simple and the fabric an easy one to sew, for me as a beginner sewer that meant 'no slippery fabrics' - I would choose cotton sateen, cotton voile, silk dupioni (just watch those pins, keep them in the seam allowances), rayon is ok if not a lot of detail. Of course it all depends on the pattern you choose - soft draping fabric is generally flattering to most figures.
If you aren't scared to try new things... knit/jersey fabrics are perfect. You can buy really dressy versions and although they seem scary, they aren't. Knit fabric is soooo forgiving - in the sewing and the wearing because it eases with the body. I love Simplicity 3503, I have made four of these dresses, two in style E (long and short) and two in style B (both short). I did have to do a muslin (I found an old knit bedspread that did the trick). It looks great on many figure types, you can do many things with the pattern and you can easily dress it up or down - I made my 40th dress in this in a silk jersey, it was so easy to make, after trialling it of course, that I did it in 8 hours.
Check out Patternreview.com. it was a best pattern of 2009 and has 57 reviews, it is popular - here is the link http://sewing.patternreview.com/cgi-bin/patterns/sewingpatterns.pl?.... Goodluck, I bought my silk jersey from Ebay but you could use ity knit, rayon jersey, bamboo, modal, lycra jersey, as long as it has a good amount of stretch. I am about a size 16 and I get loads of compliments when I wear this dress. Have attached some not very good photos just to give you an idea. I hope this is helpful and not hindering. You don't even need an overlocker, it doesn't ravel so you can leave the seams unfinished if you want and use a tiny zig zag on your sewing machine to sew it together, the trickiest part is the v neck (once you've done it once you'll be right) and getting acquainted with a twin needle to do the hem, I use lite steam a seam to do my hems and then sew them down, you don't even have to sew them if you don't want to, it sticks it together, not that I am suggesting you leave it unfinished :-).
Anyway just thoughts. Just remember to enjoy yourself. Wow, that was a long post...sorry!