How to focus and walk through menu bar on OS X using keyboard

The Alt + Letter is a Windows things doesn't appear in OS X. There are two native options to navigate the menu bar. First, very quick, and intuitive, is a Mac universal shortcut Command(⌘)+? (which is: ⌘+Shift+/) which will take you to the Help menu. From there you can search whatever menu bar item you wish, and get the shortcut if one exists; just start typing.

Second if you manually want to navigate the menu bar then, open System Preferences > Keyboard > Shortcuts > Keyboard > Check and Set shortcut for Move focus to the menu bar. This way you can move focus and navigate using arrow keys.

If you go to the App Shortcuts category in the same pane, you can also change the Help Menu shortcut to your liking.

Tertiary option would be, investing in a third party application to help you find/remember shortcuts faster. Personally, would recommend KeyCue. There are several others, just search on Google or App Store.

See the official Apple Documentation: Mac keyboard shortcuts for accessibility features

Another helpful trick:

Using your keyboard with dialogs won't work by default. Pressing Control+F7 toggles this option. The setting can also be found in System Preferences > Keyboard > Shortcuts > Full Keyboard Access.