You will need some games for one but assuming you have them.
1) Some way to transfer money. I trust paypal about as much as I trust ebay which is about as much as I trust politicians and used car sales types. Being resident computer guy also has the side effect of my opinion being regarded as worthwhile (and it seems I am not the only one).
Alas small sum money transfer (even in a common currency) is not that easy via the internet (anyone out there reading finds a way of doing it then you will be able to retire in very short order).
While I have heard horror stories from places that should know better it will likely require your site be vetted in some form (which will cost money, especially for secure certificates).
Possible solution: premium phone line, an old method perhaps but everyone can get a phone (and at worst pump coins into a phone box) and even better if I phone up it is me being a idiot if I lose out but online carries a different stigma.
Be very clear as to what is going on (avoid subscription services and state that you do) and offer links to sites dealing with abuse of those services and so forth.
Possibly even consider a sort of ticket system (whether you kick it to another line is up to you).
This limits you geographically a bit but pan european and US/Canada stuff can be set up with a bit of effort.
Either way test it several times and try to break it if you can.
2) Something to store the games on/release the games with. The probably means a nice bit of bandwidth (although if you can work torrents, other p2p and similar go for it). Usually common sense about buying the domain and a different host applies.
3) A hook. I do not know you so my cash is not going to be yours unless you give me a reason.
Demos, plugins for popular applications (including mods for popular games; some of the best coders were bedroom programmers, some of the best games started as mods......), free games, older versions of your games and my personal favourite multiplayer clients.
Sidenote scan every game/file you make with one of those online 50 different antivirus packages things, the last thing you need is to be fending off a "my virus scanner.....".
Any DRM or similar is a tough call, if you really have to keep it lean and go for say 60 days (and pump out updates) rather than the "nevar in a gazillion light years" approach that some take.
4) A community, up to you how you pull this one off. Forum software is cheap enough or even free and as much as I hate the things a blog or at least a news script is a good idea.
Sidenote this sort of software has been known to take out sites so lock it down and/or keep the money end somewhere else.
Along with this a fallback of some sort (this is down for upgrade but [link to cut down site or even ftp]) is probably worth investing in.
5) Advertising. Not my world and my generally jaded view of people makes me an especially bad person for advice on this front. The devious part of me says make something that search engines can latch onto (either name of game, tagline or general description (robbing a uncommon language of a word, mangling one or plucking something from ancient mythology tends to work well).
With the possible exception of 5) and I would consider paying someone for any premium phone stuff this can all be done for nearly nothing (and upgraded as time goes on).
