If you don't have open water swimming experience, practice that - it's quite different to swimming in a pool. Sighting is important! And if you can effectively draft the swim, you'll both be slightly faster and save energy for bike/run. If your tri has a beach start, practice that. Practice swim exits and run to T1. Practice transitions (get elastic laces so you can put your shoes on faster). Do bricks, running off your bike. Etc. I did not do a lot of these things before my first tri, and regretted it. Train all 3 disciples. There are some good plans online, but they all assume more than 5 weeks training time (most assume 12).

No matter what time you do this time round, it will be a PB. The second time you race the distance, aim to smash it.