@captin slows old outlaw <-- if you mean like that, then as @Saintly Fish says, type the @ symbol then start typing the username of the person you want to tag.
Hash tags are used on Twitter as a way to identify or start trends. Some people (including Neil and I) use them on other sites, including here, to take the mickey out of each other. They have no system functionality but are used to reinforce a point.
#Neilsalrightreallydespitewhateveryonesays