This is a fairly open-ended question. Code generation can take on a number of different forms depending on how much you want done. For simple things, T4 templates are usually good enough. In your case, I would assume you want more. Here are a couple of options for you to look through:
Entity Framework - EF will take your existing database and create a DAL for you that handles all of the plumbing. You can then use T4 templates to create ASP.NET MVC pages to handle the CRUD operations.
If you are looking for full-service code generation, you might want to try DevForce or IronSpeed.
DevForce -
http://www.ideablade.com/products/devforce-features.aspx[
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IronSpeed -
http://www.ironspeed.com/products/Overview.aspx?c=AspAlliance[
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I'm not sure about the VB.NET support on either of these solutions but really you won't be doing much coding at all if you go this far.
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