Choosing an extension
It is safe to stick to .com though .net or .org or other country specific domain extensions are also fine ones. Try to own the same domain name with different extensions to redirect traffic to your original site, rather than leaving those domain extensions at the hands of your competitors.
If someone else owns a domain that you want, do a whois checkup and find if it is in use or not and communicate to the owner about your interest in that domain name.
Avoid using hyphens, numerals or other symbols in your domain name to avoid confusion in the minds of your customers.