So I thought I would just register a .com and maybe a few more such as .net and .co.uk, but then I worry about two things:
1) If it becomes successful I might not be able to get the domain registered with the remaining TLDs will become even more expensive as the current domain owners who are just holding it will want even more money for it.
2) Phishing attempts using the same domain name with a different TLD extension.
How do others deal with these issues?
#1 is a good problem to have and unfortunately extremely unlikely. But if you do become a brand name, you should have enough money to easily purchase other TLDs.
#2 can be mitigated by filing for a copyright. US servers and Google will shutdown and blacklist phishing sites violating your brand name and copyrights.
Good luck!