Hi. Congrats on your little baby. I really love the jackets from Petco. There is one jacket that has 2 D rings on the back and you just attach the leash to those. Most jackets usually have an opening in the back. I put a harness on my baby and then just pull the D rings through opening and attach the leash. I am a bigger fan of the jacket with the D rings built in because this way my baby can wear a sweater underneath the jacket and it won't make the harness tight on her.
Usually you just watch your baby to decide when to put a sweater on them. I'm a very cold person, so usually if I'm cold, my baby is cold.
Walmart has these cute little fleece coats that my baby loves. When I show it to her and she's cold, she puts her head into it on her own. All I do is just close the Velcro. I think it might get to the point where your baby will dress herself if she's cold.
My favorite brand of clothing is the Lulu Pink brand and the Smoochie Pooch brand. One is sold at Walmart and the other at Petco. They are made from the same company. But their clothes is well made and lasts a long time. They wash well, as long as they're air dried. Although I've put some things in the drier with no problems. I would say 99% of my baby's clothes is from this line. Good luck buying clothes. I hope this helps a bit.