that's why I need to work.
]那时=过去: that's why I needed to work
I will swim at least once every month.
习惯用现在一般时态就OK了: I swim at least once a month
I've try to browse for information about your company via the internet
have try 不符合语法
browse 是没有目的,不经意的翻阅
何不简单地说:I visited your website (on the Internet) and blah blah blah....
I am really looking forward to join
I look forward to joining ... (in joining 和 to join 都是是错误的)
有人说 I look forward to 是比较正式的;而 I am looking forward to 是比较不正式的
I'll do my best in serving your company
I'll do my best to serve ... 比较自然