Oh, yes. I forgot. We are going in mid-July of this year.

Are you going on a tour? If so, the tour operator should provide such info... not much different for teen than a woman.

Are you volunteering? If so, the organization should provide such info.

- Scarf or headcovering - I always have regardless where visiting; never know when you need for a church, mosque, temple, etc. If you need to wear, someone will advise; otherwise, this doesn't take up much room in your luggage.
- Attire - generally, arms should be covered as well as legs; doesn't mean long sleeves (cover upper arm at least), but forget halter tops or any exposed skin. Same for skirts, below the knees, loose fitting as those 'peasant' types. If pants, either regular length, though crop (to ankle) are usualy fine. Remember to avoid exposed skin.
... unless staying in large cities with Western-type hotels, where you can dress a bit less conservative, elsewhere whether at hotels, cottages, guest houses... be respectful of locals.
- Shoes - comfortable that have been broken in (not new or you wind up with blisters). Ground is often uneven, so sandals not a good idea many places. Shoes with good soles and though not thought to be 'fashion forward' there are some that are... surprisingly! Flip flops work great as slippers indoors and in showers. Doesn't mean you won't find locals wearing sandals/flip-flops, but they're used to what's underfoot... you're not.
- Weather - I haven't the foggiest in Ethiopia in general or from area-to-area you're visiting, but it's always a good idea to 'think layers' where you can put on/take off as temps change during the day.

Again, as first para... check with tour operator or volunteer organization. Hope this helps.