A garage should not leak water though its walls and if it does, you need to do something about it quickly. Water penetration can soon damage property, causing major structural issues that cost a lot of time and money to deal with. What you need to do depends on where the water is coming from.
At roof height
If the water is leaking in at roof height, it may be that the gutters are blocked with leaves, causing them to overflow. Check the gutters and clean them out if necessary. Check for cracks in the guttering if leaves are not the culprit. Check for gaps between pieces of guttering if you find no cracks. Temperature changes sometimes lead to guttering expansion and contraction, causing gaps to open up between joined pieces. Refit the pieces in the joining brackets or replace where necessary.
At floor level
Check for a blockage in the drain, which might consist of leaves and/or household waste, and remove it. If there is no visible blockage but the drain is overflowing, it could be because of a blockage in a different part of the pipe work. Get a drainage maintenance company to come and investigate. Ensure there are no sodden heaps of soil stacked up against the garage wall. Make sure no water pipes at or near ground level have become compromised, causing leaks. If the garage is attached to your house, check that nobody has left a tap running.
Inspect the exterior of the walls to see whether there are signs of perished mortar. There will be holes between the bricks or stones where the mortar has crumbled and fallen out, if so. If this is the source of the water penetration, the wall needs to be re-pointed. According to The Telegraph, you should rake out any crumbling mortar before re-applying stiff mortar in sausage-shaped pieces.
Normally, brick and stone walls are resilient to rain penetration. However, an extended period of precipitation could lead to old walls becoming saturated and leaking water inside. Consider applying an anti-damp exterior wall coating or having a specialist firm do the job for you. Further, if the garage has a damp-proof course, it may be that it has broken down and needs renewing.