Can you be more specific? Do you want DIY decor? DIY invites? DIY outfit? Pinterest has a lot of good ideas to get a general idea of what decorations you think you'd like. You can DIY bouquets using artificial or wood flowers. Lots of people "DIY" their invites and Save The Dates.
DIY weddings are fun to plan and everything, but be careful because DIY weddings are hard to handle and you can have a lot of leftover product right after the wedding, as well as a lot of work you have to handle all by yourself. The only DIY project I did for my wedding was the bridesmaids bouquets and my bouquets.
Honestly I would not consider DIY unless you are already a very crafty person with a ton of patience. Your wedding is not the time to start learning skills. Also be aware that your time (figure minimum wage times the number of hours per project) and sanity equal money so not everything DIY is cheaper.
Rent as much as you can from your florist and local party rental business. Unless you plan to keep centerpieces, linens, backdrops, etc, you have to also take the time + sanity = money formula into account when reselling as well. Purchasing from Oriental Trading or Tablecloth Factory is not always the most economical when you do the math.