Yes, the best and correct way to fix a crack that's leaking is from the outside.
Noticed that I said 'leaking'. If the crack isn't leaking AND the wall isn't moving any more, then the only reason to fix the crack is for 'looks'.
If the crack leaks at any time, fix it from the outside. BUT, again, you should be sure the wall isn't moving. If the house is new and isn't finished settling, you may need to wait a number of years.
Yes, clean the crack and chisel out enough of the crack per the instructions on the can. Visit any Home Center store and ask for their products that are used to do what you need to do. SOme products work fine w/cracks that are 1/8" wide. Others work well w/cracks that are 1/2" wide. The reason you would need to chisel the crack (if necessary, per the instructions) is to give the filler material enough of the crack to 'grab' on to. Also, buy an 'expansive' filler. Fix the crack all the way down to the footing. If you don't, at some point, water will make its way into the house and you'll be doing this job, again. Do it right the first time.
My best to ya and hope this helps.
Jay J -Moderator
PS: Make sure you have the proper grading and landscape drainage all around the house. ALso, make sure your gutters are not leaking and installed where they should be. The same for downspouts. Make sure water isn't making its way BACK towards the foundation.
PPS: God Bless America!