How can i file an injured spouse form with turbo tax online?

To find the Injured Spouse section in TurboTax, click on Federal Taxes, then Other Tax Situations, then scroll down and click on Other Tax Forms, and select Misc Tax Forms, and you'll see a list of forms to choose from.  Click "Start" next to Report an injured or innocent spouse claim.  Then follow the prompts.

Remember that you want injured spouse rather than "Innocent Spouse," since they're two distinct situations.

Also, here's a link to the TurboTax support page for injured spouse claims:  https://ttlc.intuit.com/questions/1910698-how-do-i-file-form-8379-injured-spouse-allocation 

how do i allocate state and local tax deductions?

Whatever state & local taxes appear on each spouse's W-2 forms get allocated to that spouse.  If there are shared items (like real estate taxes on jointly owned property), split them down the middle.  If there are additional taxes (e.g. school district) based on income, then allocate them proportionally.