Me lastimé el tobillo y ahora camino con una muleta prestada.

Me is a pronoun that marks who is affected by the action. With injuries in Spanish, it’s very common to use this pattern:
[indirect object pronoun] + verb + body part
So Me lastimé el tobillo is literally like “I hurt the ankle (on me),” meaning “I hurt my ankle.”

The base verb is lastimar = “to hurt / to injure.”
In Me lastimé, it’s being used in a “self-affected” way (similar to reflexive usage), because the injury happened to you. You’ll often see it listed as lastimarse when it means “to get hurt.”

Lastimé is the preterite (simple past) yo form: yo lastimé = “I hurt / I injured.”
In Spanish, these -é endings in the preterite carry an accent to mark the stress: las-ti-MÉ.

Spanish usually uses the definite article (el / la) with body parts when it’s already clear whose body part it is, especially when a pronoun like me is present.
So Me lastimé el tobillo is the natural Spanish way to say “I hurt my ankle.”

Grammatically, yes, but it usually sounds unnecessary or overly explicit.
Mi tobillo might be used for contrast or emphasis (for example, “not yours”):

• Me lastimé mi tobillo, no el tuyo.
  In normal speech, Me lastimé el tobillo is preferred.

Me lastimé describes a completed event (a specific injury that happened). That’s why Spanish typically uses the preterite.
The imperfect (me lastimaba) would suggest an ongoing/repeated situation (“I used to hurt my ankle” / “my ankle was hurting”), which doesn’t fit as well here.

Ahora camino... uses the present to describe your current situation/state: “now I walk / now I’m walking (these days).”
Ahora estoy caminando... emphasizes what you are doing right at this moment. Both can work, but camino often sounds more like “this is how I’m getting around now.”

Caminar con means “to walk with / using.”
So camino con una muleta = “I walk using a crutch.”

Muleta is a feminine noun, so it takes feminine articles and adjectives:

• una muleta
• una muleta prestada (adjective also feminine)

Prestada means “borrowed” and agrees with muleta (feminine singular).
In Spanish, descriptive adjectives commonly come after the noun, so una muleta prestada is the neutral, normal order. Putting it before (una prestada muleta) is unusual and would sound marked/stylistic.

You’d make it plural:

• Me lastimé el tobillo y ahora camino con muletas prestadas.
  You can also say con unas muletas prestadas, but often the article is dropped in plural after con.