Create a new snippet:
'Tools' > 'New snippett' will open a new file. Overwrite the default text with something like this...
<% $1 %>
Typing %-tab will insert <% %> with the cursor in the erb. Add %= and %- for other erb tags.
Save the file in the erb package or in your Application Support/SublimeText/Packages/User folder with the extension .sublime-snippet