There are a couple of different methods you can use to assign Tax Codes to your Items.
You can set default Tax Codes in both your Hourly Rates Library and your Fixed Costs libraries.
If you are assigning Hourly Services to your Items, you can add a Tax Code to them by going to your Settings, and clicking in the "Tax Code" column against the relevant Hourly Rate. This will then populate the Tax Code automatically on Items you assign this Hourly Rate to:
You can also do this for Items with Fixed Prices by going to your Product / Service Library, selecting the Fixed Price Library, and clicking in the Tax Codes column against the relevant Fixed Price Item:
For Items without a default Tax Code, you can modify this on the Project Details page. Locate the Item you wish to set a Tax Code against on your Items list, and click into the "Tax %" column. For accounts not using Accounting Codes, you can set your Tax Code directly here (Tax Codes are determined by your account's Country when not using an Accounting Integration):
For accounts using an Accounting Integration with Accounting Codes, you can select an Accounting Code in the Tax % column, and the appropriate Tax code will apply:
You can also do this by clicking in to the "Acc. Code" column and selecting the Accounting Code associated to your desired Tax, that will automatically populate the Tax % column.
Please note: to apply taxes on the bill for your client, you will want to ensure you are setting the Tax Code (Out) and/or the Accounting Code (Out).