I'd second the recommendation for bib shorts. Without getting too explicit, let's just say they're even more crucial for us heftier individuals than they are for skinny riders, and are overall far superior to simple cycling shorts for everyone across the board.
Also, it's your choice if you want to wear baggy t-shirts and shorts over the cycling garb, but as you get faster you come to appreciate the slick traditional cycling garb a lot. I wore baggy stuff early on when my speeds were slower and I was cruising a mountain bike around on road tires. About three years into it I got a road bike and also made the switch to traditional road cycling garb, and really appreciated the way it cut the wind flapping and the extra drag that baggy clothes cause while cruising at higher speeds. I forced myself to get past the embarassment and inhibitions of knowing I looked really fat in my cycling clothes. At the end of the day you're doing this for yourself, not to impress anyone else, and you should do what you want and feel comfortable with. Padded bib shorts are definitely extremely helpful when out riding, whether you wear something baggy over them or not.