Ok, for starters, it's geothach and ceòthach, and IMO "th" is usually silent, so THA is at best just a strong A-sound and THU is always just a strong U-sound, but officially th is an h-sound. Just found this, it's great; notice the index by letter category across the top: https://learngaelic.scot/littlebylittle/sounds/index.jsp
Something that's key: Scottish Gaelic comes in chunks (my term).
There are slender chunks, and broad chunks.
The chunks are indicated by the vowels: slender vowels are i and e, broad vowels are a,o,u.
Slender means everything happens at the top of your mouth -- "palatized" -- your mouth is more closed, so slender.
Broad means it happens at the bottom of your mouth -- your mouth is more open, so broad.
SO if it's slender, notice where the action is in your mouth. And if it's broad, notice where the action is in your mouth.
Any consonant or group of consonants will have only slender vowels around it -- that's a slender chunk.
Or it will have only broad consonants around it -- that's a broad chunk. ("Slender to slender, broad to broad" is what the Scots say.) In the broad chunks, the consonants are kind of more normal for an English speaker -- except for consonant + h combos. In the slender chunks, really wild things happen to the consonants.
Note that the grammar shoves h into words, so words shift from, for example, caraid to (a) charaid depending on what's happening grammatically. So NOTICE when those shifts happen.
No, it's the word,side, meaning weather, that's feminine. So if "it" is referring to the weather, that translates as i. However, latha, the word for day, is masculine -- so if the rest of the conversation is about what the day is like, you might use e because it would be referring to the day.