You've asked a lot of questions in one, but I'll help you with as many of them as I can.

First, you need to add your IRC server of choice - fire up irssi, and type the following:

/server add -auto -network Freenode irc.freenode.net 6667

That will add the Freenode IRC network to your IRSSI config, and will automatically connect when you run irssi.

/network add -nick <your-nick> Freenode

That will add your nick to the new Freenode network, and will automatically use that nick whenever connecting to the network.

/channel add -auto #ubuntu Freenode

That will make you join #ubuntu on Freenode everytime you run irssi.

Assuming you have your nick registered, you can do this to identify automatically:

/network add -autosendcmd "/msg nickserv identify password ;wait 2000" Freenode

You can switch between channels with Alt-2, Alt-3, etc, with each channel being Alt-Number

If you're interested, here is my ~/.irrsi/config file:

servers = (

  { address = "irc.ubuntu.com"; chatnet = "Ubuntu"; port = "8001"; },

  #There is actually very little difference between irc.ubuntu.com

  # and irc.freenode.net - irc.u.c is just a redirect

  {

    address = "irc.freenode.net";

    chatnet = "Freenode";

    port = "6667";

    use_ssl = "no";

    ssl_verify = "no";

    autoconnect = "yes";

  }

);

# I'm a freenode user all the way man

chatnets = {

  Freenode = {

    type = "IRC";

    nick = "changeme";

    autosendcmd = "/msg nickserv identify <password removed> ;wait 2000";

  };

};



# Channels I hang out in a lot. Change these to your own.

channels = (

  { name = "#2buntu"; chatnet = "Freenode"; autojoin = "yes"; },

);



aliases = {

  J = "join";

  WJOIN = "join -window";

  WQUERY = "query -window";

  LEAVE = "part";

  BYE = "quit";

  EXIT = "quit";

  SIGNOFF = "quit";

  DESCRIBE = "action";

  DATE = "time";

  HOST = "userhost";

  LAST = "lastlog";

  SAY = "msg *";

  WI = "whois";

  WII = "whois $0 $0";

  WW = "whowas";

  W = "who";

  N = "names";

  M = "msg";

  T = "topic";

  C = "clear";

  CL = "clear";

  K = "kick";

  KB = "kickban";

  KN = "knockout";

  BANS = "ban";

  B = "ban";

  MUB = "unban *";

  UB = "unban";

  IG = "ignore";

  UNIG = "unignore";

  SB = "scrollback";

  UMODE = "mode $N";

  WC = "window close";

  WN = "window new hide";

  SV = "say Irssi $J ($V) - http://irssi.org/";

  GOTO = "sb goto";

  CHAT = "dcc chat";

  RUN = "SCRIPT LOAD";

  CALC = "exec - if command -v bc >/dev/null 2>&1; then printf '%s=' '$*'; echo '$*' | bc -l; else echo bc was not found; fi";

  SBAR = "STATUSBAR";

  INVITELIST = "mode $C +I";

  Q = "QUERY";

  "MANUAL-WINDOWS" = "set use_status_window off;set autocreate_windows off;set autocreate_query_level none;set autoclose_windows off;set reuse_unused_windows on;save";

  EXEMPTLIST = "mode $C +e";

  ATAG = "WINDOW SERVER";

  UNSET = "set -clear";

  RESET = "set -default";

};



statusbar = {

  # formats:

  # when using {templates}, the template is shown only if it's argument isn't

  # empty unless no argument is given. for example {sb} is printed always,

  # but {sb $T} is printed only if $T isn't empty.



  items = {

    # start/end text in statusbars

    barstart = "{sbstart}";

    barend = "{sbend}";



    topicbarstart = "{topicsbstart}";

    topicbarend = "{topicsbend}";



    # treated "normally", you could change the time/user name to whatever

    time = "{sb $Z}";

    user = "{sb {sbnickmode $cumode}$N{sbmode $usermode}{sbaway $A}}";



    # treated specially .. window is printed with non-empty windows,

    # window_empty is printed with empty windows

    window = "{sb $winref:$tag/$itemname{sbmode $M}}";

    window_empty = "{sb $winref{sbservertag $tag}}";

    prompt = "{prompt $[.15]itemname}";

    prompt_empty = "{prompt $winname}";

    topic = " $topic";

    topic_empty = " Irssi v$J - http://www.irssi.org";



    # all of these treated specially, they're only displayed when needed

    lag = "{sb Lag: $0-}";

    act = "{sb Act: $0-}";

    more = "-- more --";

  };



  # there's two type of statusbars. root statusbars are either at the top

  # of the screen or at the bottom of the screen. window statusbars are at

  # the top/bottom of each split window in screen.

  default = {

    # the "default statusbar" to be displayed at the bottom of the window.

    # contains all the normal items.

    window = {

      disabled = "no";



      # window, root

      type = "window";

      # top, bottom

      placement = "bottom";

      # number

      position = "1";

      # active, inactive, always

      visible = "active";



      # list of items in statusbar in the display order

      items = {

        barstart = { priority = "100"; };

        time = { };

        user = { };

        window = { };

        window_empty = { };

        lag = { priority = "-1"; };

        act = { priority = "10"; };

        more = { priority = "-1"; alignment = "right"; };

        barend = { priority = "100"; alignment = "right"; };

      };

    };



    # statusbar to use in inactive split windows

    window_inact = {

      type = "window";

      placement = "bottom";

      position = "1";

      visible = "inactive";

      items = {

        barstart = { priority = "100"; };

        window = { };

        window_empty = { };

        more = { priority = "-1"; alignment = "right"; };

        barend = { priority = "100"; alignment = "right"; };

      };

    };



    # we treat input line as yet another statusbar :) It's possible to

    # add other items before or after the input line item.

    prompt = {

      type = "root";

      placement = "bottom";

      # we want to be at the bottom always

      position = "100";

      visible = "always";

      items = {

        prompt = { priority = "-1"; };

        prompt_empty = { priority = "-1"; };

        # treated specially, this is the real input line.

        input = { priority = "10"; };

      };

    };



    # topicbar

    topic = {

      type = "root";

      placement = "top";

      position = "1";

      visible = "always";

      items = {

        topicbarstart = { priority = "100"; };

        topic = { };

        topic_empty = { };

        topicbarend = { priority = "100"; alignment = "right"; };

      };

    };

  };

};

settings = {

  core = { real_name = "Unknown"; user_name = "<your_user_name>"; nick = "<your_nick>"; };

  "fe-text" = { actlist_sort = "refnum"; };

};

If you're someone who likes to add all sorts of cool functionality, then you should take a look at the irssi scripts site - there are all sorts of little gems there.