We have about six different grammar forms to indicate the future, and "will" or present perfect are only two of those. If you are talking about scheduled or regular events, use present simple. Time clauses and if-clauses (1st conditional) use present simple If there is a sense of "maybe" or "I think so", then use "will". I have a meeting tomorrow. - I am assuming this is scheduled. You can also use "I'm having a meeting tomorrow" as an arrangement. "I will have..." isn't wrong, but see my comment above about using "will". It will start at about 10 o'clock. - The vague word "about" is more suitable for using "will". We will probably finish our session at 11. - That word "probably". :) "We probably finish.." sound a bit like broken English. If our session finishes by 11, we will have lunch at 1pm. - Basic first conditional. We don't normally use 24-hour time in English, so stay with the 12-hour format (either am/pm, or o'clock). The other options aren't entirely wrong, but they all have nuances. You need to give a specific context if you want to use them. For a basic sentence, stay with first conditional and you won't sound confused. If this still confuses you, look up "difference will going to" as a start and see how the explanations fit your context.

I have a meeting tomorrow (Use 'will have' if it was all (or most of the day) It starts about 10 o'clock./It will start about 10 o'clock. (either is OK) We will probably finish our session at 11. (different order) If our session finishes by 11 (it will finish sometime, so it is incorrect to say 'it if will') We will eat lunch at 1. No 'the', and we rarely use 24 hour times in spoken or casual English. 'We eat' is general. We will have eaten lunch by 1:30 (same)

I have a meeting tomorrow. It will start about 10 o'clock. We probably will finish our session at 11. If our session finishes by 11, We will eat the lunch at 13. We will have eaten the lunch by 13:30.