When we had professional photos of Nina done at 12 months, the photographer we used said either white & denim, or single colour clothing. In some of the photos, Nina is wearing a pale pink dress, and in others she is wearing a white shirt with denim dress. All of which I bought from Target and didn't cost too much.
I don't think I can put the link up here, but the site of the photographer we used has heaps of examples of what they've done, and Nina is in one of the pictures. PM me if you would like the link, you'll get a good idea for ideas for clothing then.
Also, I did my hair and makeup and DH & I wore nice clothes in the photos too. It just depends on what look you want for the photos.