How can I obtain a uniform bacterial sample?
You could inoculate your sample in a selective medium or an enrichment medium to kill off the unwanted species. Of course, you will need to do a literature search to find the optimum medium for the species of your interest.
Another option is to streak plates like so. With successive 'generations' of streaks, the inoculum gets diluted, so that you ultimately get separate colonies. Each colony is founded by a single bacterial cell: by picking up one such colony, you get a pure sample.
For even better results, combine both the methods described above by first growing in a selective medium and then streaking on a solid medium to get individual colonies.