Please help.
Am I correct in saying that the following phrase is future simple tense?
'if it rains tomorrow'
Future tense includes the auxiliary verbs will or shall ...
It will rain tomorrow.
We shall need to take umbrellas with us.
What you wrote is the first part of a conditional statement (if....then....), and the future isn't used ... yet.
If it rains (<~~present, subjunctive) tomorrow, I will have (<~~future, indicative) to wear different clothing and take an umbrella.
Read about Subjunctive Mood here:
http://grammar.ccc.commnet.edu/grammar/verbs.htm#mood