Payment
PayPal - it's probably PayPal that worths the most amongst simple-to-signup payment processors. There are 45 countries where you can become a Paypal seller. It's not cheap (I mean the transaction cost as signingup is free), but it's simple and straightforward.
Google checkout - only for the US and the UK.
Other electronic payment systems - just like Yandex Money in Russia, or AsiaPay in many Asian and Pacific countries
2checkout.com now offers PayPal as well as a proxy - but it's expensive and it costs you $50 to sign up. Funds can only be collected when a certain amount is on your account. But might help you out if nothing else!
COD - Cash on delivery - expensive but useful, and very simple. When the postman delivers, he takes the money as well, which is mailed to you after. Check at your post office for international options - COD works throughout the EU countries!
Local credit card payment - in many countries, local banks offer local payment options with credit card. Suprise - OScommerce and Zencart has a lot of local banking modules developed.
Direct debit - in some countries you can pay just by giving out your bank account number.
SMS and other mobile payment - check your mobile providers if they offer something for you!
Shipping
There are fast and slow, cheap and expensive shipping methods. It's a local thing that needs a lots of investigation.
You should be able to offer your customers both ends - cheap&slow and fast&expensive as well (you never know - they might need the hosiery for next day).
Planning
You will need a plan to run your business, regardless how you call it.
You will need to plan finances, scheduling, and many more. Most probably you will write a business plan. You have to! Without a plan you won't know where you are at all. It's a roadmap for you. You gonna run a business, you need a business plan! Go to a local bookstore and look for a business plan writing book.
As for software, I used Business Plan Pro, for USD 99 from http://www.paloalto.com/ - it's probably not the best looking, but it's very in-depth and very handy. Be careful, it allows to install only three times, even it's the same computer (for MS Windows environment).
Legal,tax, bookkeping
You have to know your legal status.
In many countries simple individuals are not allowed to run any business, while in others they are.
You might register as an individual business (or personal business), and there are of course many kind of corporate forms.
Same for taxation - it's your duty to find out the tax details for you as a shop owner.
Permissions and regulations! Be sure to look for the list of countries that you are legally able to sell (for example US shops can't sell to Cuba); and look for import regulations! Remember that no matter how successful your shop is, you can't keep it going only if it's 100% legal.
In many countries there are organisations (either government funded or non-gov that help with small businesses and startups - look for them, they might provide you valuable information).
Bookkeping (accounting) can be very different from country to country: be sure to look after the details.
