'Cheapest' is an objective criteria. I suggest that you take a look at these websites and research their prices: godaddy, name, namecheap, dynadot and moniker. (Check namepros for discount coupons)
I believe that you will find that godaddy and name are amongst the cheapest.
You will need to find out the freebies that comes with it. For example, if private whois is important to you,then you should add in that cost. Only when you are comparing like with like, can you make an effective decision.
'Reliable' is a subjective criteria. It is also vague. So I will assume that you are referring to support as well as security. My experience leads to a preference towards namecheap, dynadot, moniker.
As a mix between 'cheapest' and 'reliable', i will take namecheap (though I wish they will change that name - whoever who thought it up has no understanding of branding or consumer psychology).
Nonetheless you must be aware that there are vulnerabilities which you must cover by having a good internet security system in your pc, strong passwords to access your account and regular change of passwords.
Good luck with your start in this domain business.
Keep us informed, will you?
