firstly

计划旅行时，考虑几个因素。首先，决定你的预算。

When planning a trip, consider several factors. ___Firstly, decide on your budget.

我们去不了有两个原因：第一，我们很忙；第二，路途太远。

There are two reasons we can't go: ___firstly, we're busy, and secondly, it's too far.

我认为这是个好主意，首先是因为它能节省时间，其次是因为它更便宜。

I think it's a good idea, ___firstly because it saves time, and secondly because it's cheaper.