Learn Turkish: Islanmak istemiyorum, bunun için şemsiye alıyorum. - I do not want to get wet, therefore I am taking an umbrella.

Literally, bunun için translates to 'for this' (bu plus the genitive -nun, followed by için). In Turkish, it frequently acts as a conjunction meaning 'therefore' or 'that's why' to show consequence. You could perfectly swap it with bu yüzden or bu nedenle in this sentence.

Why does ıslanmak have the -mak dictionary ending here?

Just like in English ('I want to get wet'), when you want to do an action in Turkish, you leave the action verb in its infinitive form (-mak or -mek). Since ıslanmak (to get wet) is the action you don't want to do, it sits right before the main conjugated verb istemiyorum.

Wait, shouldn't the negative of istemek be istemeyorum? Where does the 'i' come from?

This is due to a rule called vowel narrowing. The verb root is iste- and the negative suffix is -me. However, when the -yor suffix is attached, it forces the wide vowel (e) right before it to narrow into an i. So, iste-me-yor-um naturally becomes istemiyorum.

Why is it just şemsiye and not şemsiyeyi if it's the object of the verb?