Hi Christina and welcome to the forum,
I think Kamran has given you some good advice there. The only thing I would add is if you're new to the business, maybe don't jump into developing your own website straight away, use platforms like eBay offer. Now ultimately, I think developing your own website is the only way to go if you plan on being in this business for a long time, but it's not an easy thing to do and marketing it effectively certainly isn't inexpensive, so it's a lot to take on for a new seller just getting started.
Regarding CSV files, your supplier will or should be able to provide you with CSV files for upload. So just make sure that you confirm the suppliers that you want to deal with provide the CSV file for you.
There is a stack of information on dropshipping spread throughout the forum, so it's most definitely well worth spending some reading through to bring yourself up to speed. You're going into a bit of a lions den, so the best advice I can give you is to be well educated on the subject.