What is a good hosting service for me to use? If I'm developing the site does that mean I choose the hosting provider?
We use
http://rackspace.com (top notch) in the US and
http://webhosting.uk.com for the UK (although we recently found out they are a reseller of a reseller

). However, to answer your question.. ask your client what they want. It's their buck, so they should decide whether to trust your decisions or if they want to go with their own hosting.
What should I do if the site crashes or has problems sometime after I've finished the work? Will I always be the go-to guy for all the people I contracted with? (This question is high priority)
Can't speak from personal experience, but I think the industry standard is to provide ~1 year free support and updates (which can be optionally extended or service upgradable) through phone and/or email and/or messengers. What kind of support the customer can expect can affect their decision significantly, so you want to make the customer feel like they won't be alone when something goes wrong. Using Swift Mailer as an example of supporting an application: Chris was able to service a large proportion of the users through basic (free) responses through devnet forum and their own mailing list (although free means at the developers own dicretion when/how to reply). However, I believe he also offered an enterprise support for his paying customers. So again, it really depends on the context and whether there is more benefit servicing your users in a large scape, or a private/focused scale.
He needs to pay the for hosting and domains right?
It's a pain registering someone elses info for them. It takes 5 minutes to register a domain, and if they do it themselves you do not need to collect their personal/financial details in the process. Because after all, they probably want everything in their name, not yours.
Is it better too charge hourly or commission only?
This is a very vague question and is difficult to answer without context. Generally speaking, if your hours per week are limited then you would likely be paid by the hour. However, if your project is on-going or involves some personal stake, then you're probably better off being on commission/salary.