elster.de is the official means to input your tax info supported by the tax office. However, it's largely an online form with some legalese and dry formal explanations in a sidebar.

You can largely use it if you know what you are doing or have the most simplest tax case, but it won't offer you suggestions how to optimize taxes, and doesn't include large lists of items that you could possibly deduct. This service is offered by proprietary tax tools like WISO or QuickSteuer which offer questionnaires trying to cover more details of your tax situation to find out opportunities to optimize, and include large help files that contain examples and explanations.

Cost for these proprietary tools can be very low, they go on sale for like €5-€10 at discounters during tax season or are like €20 for the fully featured versions.

These tools also manage items like long-term deductions over the years, so that's their lock in effect.

There's Elster.de