Excellent! I just wear whatever but never something new for a long run. Make sure whatever you wear in the day has covered hundreds of miles with you. For me it's standard cotton everyday leggings and a cotton t-shirt and a really cheap florescent running jacket if it's cold. The only exception to forking out is to get the best sports bra and break it in gently. So 5 k first and if it's ok, build from there.
In terms of training, I would want to be doing a half comfortably before starting a three month marathon training schedule. So depending on when it is you may only have a couple of months to get there. So I would do something like this.
Week 1: three 5k runs plus an 8k
Week 2: three 5k runs plus a 10k
Week 3: three 5k runs plus 12k
Week 4: three 5k runs plus 12k
Week 5: three 5k runs plus 14k
Week 6: three 5k runs plus 14k
Week 7: three 5k runs plus 18k
Week 8: three 5k runs plus 20k
That's hugely challenging but I would think about intervals and speed on your short runs but forget about it on the long runs. Take it really, really slow on those. Just aim to get the miles in.
Then I would look up a beginner marathon schedule to cover the three month build up to the actual event. Again focused only on distance, not time. I think you have to see your challenge as completing it, not aiming for a time.
Fall in love with running if you can. I was obsessed! Anything to do with running. I read books, magazines, online. I was fascinated by kit and extras (e.g. gels). I thought about little else.
I also did loads of yoga. Always after running as part of my cool down stretches but also additional hour long sessions.
You will never be stronger, mentally or physically than when you are marathon ready.
Don't let anyone tell you you can't do this. You absolutely can.