New to DB design

So I'm making the design for an application system and not sure if it's appropriate to create a new table.

Basically we have the client and all his base information. then we have other things such as his assets which if made into their own table would just be 1 line per applicant but if it's put into the general client table there may be many nulls and yet Normal Form is not violated in either scenario.

Any one have any good rule of thumb for this kind of thing?

Okay brainstorm with me /prog/ do I want an individual table for each school and then have a sub table saying which school each kid  is being assigned to. The benefits of that is that it is a speedier look up however it's more taxing when filling out the application as when someone types in and submits the school, will have to do a look up if the school exists get the id number and then assign it if it does if not I have to crate it. This also allows for much more expandibility as I can reference each school and it's assets by creating separate tables linked with the school.
 On the other hand if I just keep child and school information on the same line insertion is easy and performance look up times shouldn't be anything remotely noticeably different, but I do lose the ability to categorize the schools in any effective matter.