"Me voici à..."
This literally translates to "Here I am at...". You can then follow it with the place you are in.
For example:
* "Here I am in Paris" would be "Me voici à Paris."
* "Here I am in the garden" would be "Me voici au jardin."
You can also use:
* "Je suis ici à..." which means "I am here at..."
* "Je suis dans..." which means "I am in..." (this is generally used for enclosed spaces)
These phrases are all grammatically correct and will be understood by French speakers.