if they are single stem type (not a bush variety) then make sure to remove all side shoots.and nip out the growing tip after three or four trusses have formed, if there are any more the top truss will probably never ripen anyway.They need sun and lots of water. Also more 'leaves' sometimes grow on the end of the fruiting trusses and these also should be cut off as they will take the strength from the fruit so will get small tomatoes I am assuming these are outdoor plants not greenhouse ones