I'm not sure why you got the idea that you cant' use the future tense in an "if" sentence, since most conditionals deal with potential future events. So it's fine to say:
If you will/won't come to my party ...
If you will/won't buy groceries this evening ...
If you will/watch that movie with me ...
And so on. It is not correct grammar to use "don't" in the future tense, although you may express some kind of present action that affects something in the future:
If you don't want to come to my party tomorrow ...
If you don't have time to buy groceries this evening ...
If you don't feel like watching that movie with me ...
Lastly, "let me know from now" is not natural English. Instead we would just say "let me know". Examples:
If you won't come to my party, please let me know?
If you won't buy groceries this evening, please let me know (so I can buy them this afternoon)
If you won't want to watch that movie with me, let me know (so I can ask someone else).