Get a new meter can installed, a 320 amp with double taps (this will include a new mast down to the can, new weatherhead and new cable from the transformer to the meter (service entrance cable) The power company will at least replace the cable from the transformer to the weather head, and possibly all the way. The meter will be mounted up higher on the house. Underneath the meter will be two disconnect boxes with 150 amp disconnects, one would feed the house's existing panel, the other would run (underground?) to the garage.
If you have 100 amp in the house now and get along with it, and a small house, 150 amp in the house should be large enough for future expansion.
You could go larger (400 amp meter can and two 200 amp disconnects) but I doubt you really need it.
Edit: Forgot to add, if your existing wire from the meter to your breaker panel is limited in size, say to 100 amp capability, you would either have to upgrade it also, or simply put a smaller disconnect supplying the house. You could, at a later date upgrade the service entrance wire to the panel and put back in the larger disconnect.
Charles