In general starting from zero you'll find some customers somehow, even just randomly and then you have to be awesome for them and build from there. The level of churn and flux at the low end of hosting is actually pretty high. You could put up little more than an unbounce page and probably get signups with credit cards at a trickle volume.
For us at Slicehost, we had an amazingly fortuitous start, due in equal parts to strategy and luck. We saw pretty clearly that rails was picking up steam really quickly and the hosting options were pretty shitty (shared hosting at the time didn't support the versions of ruby and rails people needed not to mention the memory hungriness of the framework and dedicated was still fairly pricey with 100-200/month being dirt cheap).
So we picked a really ripe initial niche market to spend time making ourselves visible in, which we did in forums, chat rooms, etc. The luck came in that we got some pretty vocal early customers who all had a great experience and evangelized us. That was lucky because either of those factors could have easily gone the other way. They could have been quiet customers or we could have had early blips in service (we had plenty of later blips, we just had a nice patch of initial smooth sailing).
