The second one is correct. You cannot use 'discuss on' in English.

Both are correct, but the first is implying a few words: Let’s discuss on 'how we do/go' the way forward tomorrow. More common to be explicit, so not too often you'll hear your first version.

Thanks Denis!

You don't need to use the word "on" with the verb discuss, which already means" talk about". You discuss the topic. Let's discuss the way forward tomorrow. Correct. Let's discuss on the way forward.....the use of "on" is redundant, but not absolutely incorrect. Sometimes when speaking people choose to be redundant in order to stress their point, so in the sentence" Let's discuss on the way forward tomorrow.", the speaker could be using 'on' in order to stress the point . However with the noun discussion, you can use on. For example, Let's have a discussion on the way forward tomorrow.