After the word "if," you usually use a present tense verb to talk about the future. The part of the sentence that contains the word "if' is the dependent clause and the other part of the sentence is the independent clause:

If you see something nice for Joan, please buy it.
If you see something nice for Joan (dependent clause)
please buy it (independent clause)

Examples:
▶ If the weather is nice tomorrow, let's play tennis. (NOT: If the weather will be nice tomorrow)
▶ If you are ever in Boston, give me a call. (NOT: If you'll ever be in Boston)
▶ If we have time tomorrow, we'll go to the Metropolitan Museum. (NOT: If we'll have time tomorrow)
▶ If I win the lottery, I'm going to quit my job. (NOT: If l will win)

Exceptions:
=> Use if + will (or the contraction 'll) for polite requests:
▶ If you'll follow me, I'll show you to the restroom.
▶ If you'll be seated, we'll start the program.
▶ If you'll turn off your cell phones, the performance can begin.

=> Use if + will (or 'II) to emphasize that you'll do something IF it will achieve a certain goal:
▶ If it'll make you happy, I'll organize the party.
▶ If it will make Chloe stop crying, I will give her a piece of candy.

=> Use if + will (or 'II) for indirect statements. Indirect statements start with clauses like "I don't know if'' and "I'm not sure if":
▶ I don't know if I'll be there.
▶ I'm not sure if I'll go tomorrow.
▶ Do you know if he'll be at the party?
▶ Who knows if I'll get an interview.