Make sure you have read the legal bit on my home page.
For people and animals, presence or existence uses the patterns:
NOUNgaPLACEniimasu.
PLACEniNOUNgaimasu.
You can also make the PLACE the topic:
PLACEnihaNOUNgaimasu.
imasu is the present/future polite form of the existence verb iru, which (rather surprisingly) is group 2 and regular.
ga indicates the grammatical subject in those cases where it is not the topic.
ni has several uses; here, it indicates a place where nothing is happening (simply being present does not count as something happening).
Examples:
saburinasangashyokudoimasu. "Sabrina is in the dining room."
shyokudoniwarumonogaimasu. "There are ruffians in the dining room."
Negations imasen, past tenses imashita and negated past tenses imasendeshita are all straightforward.
The above assumes that no great differences of social status are involved.
Where the being which exists is clearly superior to the speaker (e.g. is a wizard, or is a human if the speaker is a robot), use ira,shyaimasu rather than imasu. ira,shyaimasu is the slightly-irregular present/future polite form of the respectful existence verb ira,shyaru.
Example:
shyokudonimahoutsukaigaira,shyaimasu. "There is a wizard in the dining room."
Where the speaker is commenting on his own presence or existence, or that of a close friend, to someone clearly superior (e.g. a ruffian speaking to a wizard), use orimasu instead of imasu. orimasu is the present/future polite form of the humble existence verb oru.
Example: tomodachigatakusanorimasu. "Lots of my friends are here" (as said by a ruffian to a wizard).
Again, it is bad form to state as a fact anything which is deduction or appearance. One can see by looking that Sabrina is in the dining room, so stating this as a fact is fine. But is it a fact that there is a wizard in the dining room? He may be dressed as a wizard, but deducing from the fact that someone is dressed in in flowing robes and carrying a staff that he is a wizard is supposition, so it is better to use soudesu until such time as he actually casts a spell. To connect this to verbs, use the pre-masu form:
shyokudonimahoutsukaigaisoudesu. "It looks as if there is a wizard in the dining room."
warumonotachihamahoutsukaigaarisoudesu. "The ruffians appear to include a wizard."