This link should be quite helpful: http://esolangs.org/wiki/brainfuck_algorithms
It contains algorithms for multiplication and also an IF condition as well as boolean comparisons (to check if, for example, the user pressed enter [character 10] to end the input.)
Then what you do is this (I will write some pseudocode and then it's up to you to implement it using the algorithms described there). I will tell you also give pseudocode on how to implement a while loop at the end since that is not included in that page (but pretty simple nonetheless... relatively). You will definitely be amazed when you manage to understand exactly what each character is doing :D. Anyway, here goes:
you need two cells A and B
move to B
input a character
while B is not equal to 10 (the newline character) then
subtract 48 from B ('0' is character 48, so if we subtract 48 from any digit entered we should get its value. Of course this assumes that the user only presses digit keys or enter. I'll leave it as an exercise to you to do error checking)
multiply A by 10
add B to A (you can just move B to A like this [<+>-] since you will not need B's value anymore)
move to B
input a character
And here's a bit of info about how to create a while loop. Suppose you have this code: while (condition) {body}. I will assume you managed to implement the code for the condition using the link I gave you earlier. You need a cell in which to store the result of the condition, which I'll call C
execute condition and store result in C
start loop using [[-] (start the loop and immediately clear C)
execute loop body
execute condition and store result in C
end loop using ]
