The first sign I was pregnant was sore breasts. They grew, I bought new bras, which I promptly outgrew. For the first trimester, just get some inexpensive sports bras at Walmart. After that, try to stick to inexpensive solutions until you get to the nursing stage.
For the same reason, avoid pre-buying maternity tops. I got a lot of cute ones in preparation during the first trimester. What fit my chest then sadly no longer fit in the later two trimesters. Just wait until you actually need it and then go get the correct size.
Maternity jeans are awesome and very comfortable, but there is more involved in pulling them up and down, which you do a lot with a baby bouncing on your bladder. I got a few pairs of comfortable pajama and lounge pants a couple sizes up at the thrift store for wearing around the house.
Don't buy anti-nausea solutions or any other pregnancy symptom helps until that symptom actually appears. A lot of symptoms you hear about you may never develop.
Pregtastic.org podcasts are a joy to listen to when all you can think about is pregnancy, and no one else is interested in your new obsession. I listened to all 200+ in the first trimester. Pregnancy forums are also great, I use BabyCenter.
Take everything with a grain of salt and do your research. Not supposed to eat unheated deli meats? When you research the actual risk level to you and the baby, you may decide that it is crazy low enough that it is OK to go head and eat a cold sandwich now and again.