1) What you should do to prepare for your trip - the most important thing to do is relax and get as much rest beforehand as possible so you are well rested and ready to go when you land.
2) As far as clothing goes, bring layers. Paris may be hot but it may also be cool, especially at night, with it generally getting warmer the farther south you go.
My feeling towards clothing is to wear what I would typically wear at home, I don't buy specific or special travel clothes etc. I'm heading to London and Paris for 10 days in late September and I've already started to sketch out my wardrobe, the weather will be a bit cooler for me, but you can use it with Rick's Women's packing list to plan. I plan on bringing 1 fancy dress, 1 casual dress and 1 dress in between that with a change of accessories can take me from day to night. 2 pairs of premium dark wash straight leg jeans, one pair of dress pants, one black skirt, and then a mixture of 2 tank tops, 2 button ups, 2 t-shirts and 3/4 cardigans to mix and match in neutrals. Light weight summer scarves and accessories help change things up, and a trench style jacket.
Shoes: 1 pair of black day flats, one pair of dressy flats and a pair of calf length boots (you should sub sandals)
Hope this helps, have a fantastic trip!