1. I'm leaving at the end of July...我要在七月底动身.……
at the end of意为“在.......的结尾;在.......的末端”。如:At the end of the year，we had a final exam.
在年末，我们进行了一次期末考试。

2.If you've already paid for the air tickets and for homestay，itshould be OK.
如果你已经付过机票钱和家庭寄宿费了，那就应该够了。pay for表示“支付......的费用，为......付款”。如:去年暑假我爸爸支付了我学车的费用。
My father _paid for
my driving lessons last
summer holiday.

3. You'd better ask the travel company about the total weight forall your bags.你最好询问旅行公司关于你所有包的总重量。had better意为“最好”，常缩写为'd better，后接动词原形。常用短语:had better do sth.,意为“最好做某事”。如:你看上去累了，最好休息一下。
You look tired.You'd better have a rest.

4. Oh,by the way，don't forget to take your passport.噢，顺便说一下，别忘了带你的护照。
by the way意为“顺便说一下，顺便问一下”，用于转入与刚才说的主题无关的事。如:
顺便问一下，你喜欢什么颜色?
By the way ，what colour do you like?