What is your signal like outside of the parking garage? A cell booster can boost what it can pick up right so if the signal outside of the garage isn't great, it may not help you much (in addition to johnb35s comment about metal and concrete).
If you do decide you want to go down the booster route, you can do some preliminary scouting by checking something like Cell Mapper and locating where the towers are -- make sure you narrow down to your carrier/supported-bands, otherwise you're just going to help everyone else
As an anecdote, I have really bad reception where I am (-116db to -120db) and after putting the booster
very roughly in place (just plopped inside my garage actually), the service is much improved (-85db to -105db). I just bought a basic residential unit and roughly eyeballed placement of the antenna. I had a rough idea which direction the cell tower was in so I had it point that way. When I get around to installing it outside, I expect it to get better still.
Make sure to go for the thickest (read: more expensive, less flexible) wire though, especially if it is a long between the two endpoints. The signal loss between different types of cabling is no joke: I originally started with 60ft RG6 and then bought some cheap 100ft RG6 for it to be almost unusable. Switching to 100ft RG11 was a massive improvement. There are fancier types of wire available -- if you're going with a professional installation, the pros may be able to suggest whether or not it's worth it.