When a sentence is written in passive voice, the Object of the Verb of the active substitution of the sentence is moved to the Subject position in order to emphasize the Object, as in "Computers will be operated by you."
This sentence empahsizes the Object computers of the active sentence "You can operate computers" in order to change the dynamic or the relationship between the actor/agent ("you") and the recipient of the action ("computer").
The construction model for a passive sentence is Subject, auxiliary be + Past Participle, Object as in "Computers (S) will be (auxiliary be, simple future tense) operated (PP) by you (O)."
To substitute a passive voice construction for the above active sentence "I just had one doubt," you will follow the above model with the result: "Just one doubt is what/all I had," with "Just one doubt (S) is (auxiliary verb be, present tense) what/all I had (O)."
To substitute a passive voice construction for the active sentence, having an omitted Subject you, "So, [you] please help me as fast as you can," takes a little more work.
Firstly, recognize that "as fast as you can" is an Adverbial of Manner and not part of the active/passive model, rendering the central active sentence as "[You] help me," with "You (S) help (V) me (O) + as fast as you can (Adverbial of Manner)."
The substituted passive construction will be "So, I will be helped by you as fast as you can, please," with "I (S) will be (auxiliary be, simple future tense) helped (PP) by you (O) as fast as you can (Adverbial of Manner), please." You might notice that this syntax is a little awkward as it loses its imperative function and becomes more of a pleading prediction, which is why it is said that imperatives cannot be substituted with passive voice construction in English.
The tense of a passive sentence can vary widely using is, was, will, been, will be, am, to be, will have been, are, were, were being, among others. Though different, since be is an irregular verb, they are all the same in that the passive verb is auxiliary verb be. For a more complete view of the tense variations of passive verb be, see EnglishClub.com.
The sentence you have given us, Help me, is a command (it is in imperative mood), and imperative sentences cannot be transformed into passive voice.
Other sentences that cannot be transformed into passive voice are those with intransitive verbs (verbs with no object): I went dancing last night.
There is also a list, in the link below, of a few other common verbs that cannot be cast in passive voice.