"if you want to..."

How do you combine "if" with wanting to do something?

For example:
If you want to drink, pick something up on your way.

If you want to go, please call Sally.

There are a few different ways to express this "if...". You can start the sentence with or without "moshi" (if).
For the verb, you start with the "...tai" (want to do...) form, and either:
(1) add "... (no) nara"
(2) change to "..takattara."
(3) change it to "...takereba."

If you want to drink something, pick something up on your way.
- (Moshi) nani ka nomitai (no) nara, tochu de katte kite kudasai.
- (Moshi) nani ka nomitakattara...
- (Moshi) nani ka nomitakereba....

If you want to go, please call Sally.
- (Moshi) ikitai (no) nara, Sarii ni denwa shite kudasai.
- (Moshi) ikitakattara...
- (Moshi) ikitakereba...

For the slightly different nuances among the different verb forms, please refer to some grammar reference on "tara," and "...kereba" forms, etc.

Personally I find the first two more natural in these sentences.