Does my Internet service provider know when and if I download Tor? How could I prevent that?
If you use your ISP's default DNS servers then it's practically guaranteed that they'll know you've been on the Tor website. If you make a connection to the Tor website that just happens to download the exact same amount of data as the Tor Browser Bundle then it's possible for them to tie the two together.
If you're in a region where they're likely to care about that then it could be an issue. It's worth looking at this question for ways to download Tor without your ISP knowing (which I won't repeat here).
If your region is more open then you can take simpler steps like changing to alternative DNS providers. There are still ways your ISP could track the sites you're visiting but it won't stand out as obviously so that may be enough for you.
Note that there are two dimensions to this question: What is technically possible for your ISP and what is legally possible. In many jurisdictions, it is illegal for ISPs to spy on their customers and log their actions even though technically they may be in a position to do so.
This interactive graphic produced by the EFF provides an overview of what is technically possible.
If you are using HTTPS to download Tor (recommended!), the ISP be able to see the that you are connecting to the server that hosts the Tor website. They will also see roughly how much data you are transferring. Thus they can guess that you are probably/possibly downloading the Tor Browser Bundle.
If you already have a version of Tor (for example, if a friend gave you a Tails CD) and you use that to download the Tor Browser Bundle, then the ISP would not know that you are connecting to the Tor website. They would still see roughly how much data you are transferring, but that information alone is likely to be inconclusive.
As others have noted, unless you tunnel your Tor traffic through a pluggable transport or similar, your ISP will still know that you are using Tor.
It is possible that your ISP knows that you download Tor.
You can prevent your ISP from knowing by downloading Tor through a VPN.
In my opinion, which is not necessarily correct, even if you use HTTPS, ISP might be able to know that you downloaded Tor by looking at the URL you accessed.
You could also consider using a router that has Tor embedded in it like a PAPARouter. You can anonymize several devices and as a bonus, PAPARouter by default does not use US or US intelligence friendly exit nodes. Good luck!