I would like to add some intricate points to snailboat’s answer.
Snailboat is right in that in general, verbs in Japanese do not have to have a subject, even an implied one. Also it is right is to say that we cannot use ～ことにする to state someone decides that someone else will do something. ～ことにする is used when someone decides what he/she will do.
However, there are cases where BがXすることにする sounds fine to me.
As you might be aware, the sentence
is unnatural. However, if there is some context where who goes outside is important, then it makes sense to say:
(2) 私は自分が外へ出ることにした。 I decided that I would go outside.
For some reason, it is strange to repeat the topic and the subject as in (1) even when who goes outside is important.
As another example, suppose that a team of athletes discussed the running order in an upcoming relay race. Then it is fine to say:
(3) 話し合いの結果、康介がアンカーを務めることにした。 As a result of the discussion, they decided that Kosuke would run as the anchor.
The subject of 務める is 康介 and it is explicitly stated. The semantic subject of 務めることにする is the team, and it is not explicitly stated but implied by the context.
(I think that there is another interpretation of (3) depending on the context. See the remark after sentence (5) below.)
If we say
then I think that it means that Kosuke was not one of those who decided it. Probably he was not at the discussion, or he participated in the discussion but he did not agree to the decision.
By the way, if we change が to は in (3) and say
then it is more likely to mean “As a result of the discussion, Kosuke decided to run as the anchor.” That is, 康介は specifies the topic of the whole sentence rather than just the subject of the アンカーを務める part.
It is also possible to interpret 康介が in (3) as the subject of the whole アンカーを務めることにした part. In this case, (3) means roughly the same thing as (5), with an emphasis on the subject: “As a result of the discussion, it was Kosuke who decided to run as the anchor.” Therefore sentence (3) may be ambiguous depending on the context. But given that it was a result of the discussion, I think that it is more natural to interpret that the team decided it unless the context implies otherwise.