Signal weak - Star Samsung Cell Phones

I'm assuming that your cell phone works fine.

Your cell phone gets its signals from cell towers serving your cell phone service provider, e.g. Sprint, AT&T, Verizon, T-Mobile.

If you are in a building or elevator, your signal can be weak. Try going to the edge / side / top of a building, like by a glass window to see if the signal is stronger.
Or if your cell phone service provider (phone company) doesn't have a cell tower close to where you are, the signal can be weak or nil.
You can find out your cell phone company's cell tower coverage areas ...by checking their website. If you are at locations where your cell phone service provider has good coverage, you will get stronger signals. Otherwise, expect to get weak signals.
Yes, it's always prudent to check which cell phone service provider has good cell (tower) coverage in areas where you are (using your cell phone) most of the time, and subscribe to that company's cell phone service.