Read and write data, but not being able to alter any objects? In that case do

ALTER ROLE db_datareader ADD MEMBER [domain\user]

ALTER ROLE db_datawriter ADD MEMBER [domain\user]

or, if you are on SQL 2008 or earlier:

EXEC sp_addrolemember db_datareader, 'domain\user'

EXEC sp_addrolemember db_writereader, 'domain\user'

You first need to add the user to the database as per the previous post.

As I typed these commands from memory, I advice you to check the exact syntax in Books Online.