A freeroll tournament is tournament poker with no entry fee. The prize pool for such an event comes sponsors and/or the host. Poker sites offer freerolls as a gift to their loyal patrons and as a promotional tool as well.
Freerolls have much lower prizes than professional tournaments. Worse, there are often thousands of players signing up and only a small portion (say the top 20) may get paid anything. But they are free and provide good practice for poker amateurs.
Early Stage
Playing freeroll is like other tournament poker play. But here since it is online poker, there will probably be lots more fish around than in a live tournament. Stick to very strong hole cards in the early stage. The logic behind this is simple: even if you make a hand with your low card and high kicker, someone else can easily beat it with a better hand. Suppose you have A9. If you hit 9 pair on the flop with A kicker, there could be someone there with a pair higher than 9. If not, they can wait up to the river since it's the start of the tourney and chips are a-plenty. More players can afford to play more hands, so the chances are you can be beaten. Play tight for now.
Beware of calling all-ins. Players like to make them in freerolls. Don't call unless you have a very strong hand or you know that player is apt to call even with just a pair (then you wait til you have better than a pair).
Do not bluff early in the freeroll tournament. There are far too many loose callers around at this stage. Bluffing works much better when play is tighter.
Read the board before you go wild with your hand. If you hit a high pair (like KK) on the flop, don't forget to check what the other cards are. If there is a sequence like KQJ or AKQ, someone could have a straight, or a two pair since lots of players like to play high pairs.
Middle Stage
Now you want to retain what chips you have earned while still accumulating new ones. Play more aggressively but don't be stupid.
As the tournament progresses and the blinds go higher, you will need to bluff. Bluffing is necessary because if you just keep folding waiting for that ace pair, you will lose chips just posting the blinds when your turn comes. With bluffing, you can win even without solid hands. Start to bluff from late position when play is tight. But don't bluff your way to every pot. And NEVER bluff with minimum bet! Go bigger.
Final Stage
Bluffing is most crucial here if you are short-stacked (and you probably will be). Don't bluff the big stacks. Pick on the smaller stacks you know are tight. And steal blinds when you can.
If a short-stacked player goes all-in and you have a good hand like a top pair with a high kicker, call them.
Use reputation to your advantage. Hopefully you will know at least some of the players at the table, and they you. If you were tight early on, that should help you bluff. Likewise if you know other players are tight or loose, bet accordingly to them.
If you are on the button and no one else has called the blinds, raise. You will succeed in stealing the blinds at least some of the time.