Risk is an easy game to learn but true mastery can take years. Fortunately there are some basic tenets to follow that will definitely give you the edge when playing Risk online with other less skilled players.
1. Never attack with less than three troops. Four is the minimum amount you need to roll with three dice, meaning your best odds as an attacker. If there is a life or death situation, and you need to attack a 1 with 2, then do so. But in normal play, hold back when you get down to three.
2. Do not extend yourself to take a bonus unless you can defend it. I see it all the time. New players eager for some extra troops manage to cling to some continent, but then have weak defences. Not only will you not hold the bonus, but it makes you a huge target as you likely have 1's or 2's on the various regions, making for a quick sweep from a large stand on your border.
3. Only make a few moves in the beginning. Winning a game of Risk is not done in the first round. It takes patience. Often new players will make several attacks in the first round, trying for a quick land grab. Even if they are successful, they become open to stronger predators in the area. Don't worry about getting a card! It's just not that important in the early game. It's saddening to see a player wailing 2 vs 2 or 2 v1 in order to get a card. In fact it is often beneficial to not make any moves in the first round, or maybe just a reinforcement.
4. Do not leave 1's lying around. A single troop is easy to kill and people will not hesitate to do so. As soon as you add another troop to make two, then their defense is doubled and players will often think twice. An adjacent defence of 3 and 1 would be stronger as a 2 and 2. The only times you should leave 1's is if they are in your interior lands.
5. Have a plan. Decide early what you want to do and focus your efforts there. IF possible use diplomacy to clear the way a bit. Others players don't want to be in your way if it's not in their plan. If you wait to long too see how the board plays out, your plan might as well be useless as you don't have a foot hold anywhere.