you do need a freshwater fishing license in RI if you take herring upstream of any first bridge from the ocean and your limit is 24 per day. no taking of alewives is allowed on sunday, monday, or tuesday.
no licence is required or limit is imposed on the ocean side of the first bridge.
as for the how to get them...
most people use dip nets. basically you put your long handled net into the run and try to scoop the little buggers. runs do differ and so different techniques may be used. i'd suggest that you just go to a local run and see how folks at the run are gathering them.
keeping them alive is a whole nuther ball game. just do a search on livewells for more info..
oops...just noticed your from mass
Some local towns require permits, and have different restrictions reguarding the taking of herring, check with them.
i am pretty sure no freshwater license is required for taking alewives or herring.
for more info check mass dmf
http://www.state.ma.us/dfwele/dmf/index.html