Dear Phoebe, firstly you can increase number of bacteria using an enrichment medium then you can isolate your desired bacteria in a selective medium.

Best regards.

Dear Phoebe, 

If you are just interested in isolating the bacteria and if you are not quantifying the communities you can put the soil in peptone water and put in a shaker some hours and then plate it in a selective medium. Use shaker+temperature to obtain a faster growing.

Zayda

Dear Phoebe,

As suggest by the earlier answers @ Zayde and @ Mark, you will need to selectively grow the bacteria of interest, however, the selective medium/media to use depend on which bacteria you are interested in isolating. since you have not stated the bacteria we may be able to suggest any such media.

Additionally, the selective media may allow the growth of a few other bacteria including the bacteria of interest, therefore a knowledge of the colony morphology  may be useful in differentiating the bacteria of interest from the others that may grow along with them.

You will have to plate a serial dilution of the peptone water soil wash or the enrichment medium culture, so as to have single well separated colonies, which should be allowed to grow for a longer time in order that the different colony morphology be clearly identifiable. 

An enriched media as brain heart infusion (BHI) is useful for enhancement of bacterial growth and increase their number. To avoid increasing the number of contaminants, you can make subculturing from BHI to an enrichment broth then  isolate the desired bacteria on a selective agar media in pure culture.