On my iPhone...
If you want to reply to a post hit the upper right hand "Text bubble" to reply. If you want to quote something, tap the post you want to reply to, it will turn blue. Then hit the bottom right arrow that points to the left, it will open a window of choices, tap the "Double Text icon", then tap the upper right bubble just like you would if you were to reply without a quote.
To make a new post, go to the forum you want then select the top right icon and it will give you some choices. One will be NEW POST.