Branding
Name
For idea validation you just need some name, nothing super cool and amazing, don't waste much time on it because if you product will take up most probably you will change and for idea validation name is not that important.
There are 8 types of business names and I would recommend reading this great article and also take a look at this infographic . Try to come up with something quickly and don't spend too much time on it. If your company takes up you will have enough time to come up with something cooler.
To check name for available domains and social accounts you can use Startup NameCheck. Also you can use Instant Domain Search for checking domains and generating ones.
Optionally register name on Namecheap or some other service and you are ready to build landing page now.