Five card draw is the oldest and simplest form of poker. It is very straightforward to play and the online version can be quite profitable as people are not easily able to read your reactions as they are in person so when you are dealt a good hand there is little or no chance that anyone will know it unless you keep up some kind of commentary in the chat window.
Since you are limited to only five cards and none of the other players will see what they are until the end of the hand there are only a few strategies that can be employed to make the most of the hand you are dealt. A good starting point with online five card draw is to keep a notepad handy to track of your opponents draw strategies versus the hand they end up with as you will be able to learn about the likely cards they are holding once you know how many cards they draw versus the end result in their final hand.
If you are playing with four or more other player then it is probably not worth continuing a hand if you don't have at least a pair in the first five cards you are dealt although an almost flush or straight might be worth keeping. Because there are only two rounds of betting in five card draw the betting in the first round tends to be more intense than in other versions with multiple rounds of betting so unless you have a good hand, staying in could cost you.
In most cases the winning hand in a five card draw game will be either two pairs or three of a kind. This does not exclude the possibility of there being higher hands out there but if you are holding three of a kind by the end of the card exchange then percentage wise you will have a good chance of winning.
Watching how people exchange their cards will give you a good indication of the kind of hands you are up against. If your opponent draws three cards they are likely holding a pair, if they draw one they may be going for a straight or a flush. If the player drawing three cards made a big raise before they drew their cards then they likely have a high pair. In the case they draw only two cards it could be a bluff to convince you they have three of a kind or are doing a 'Hail Mary' to try and complete a straight or a flush.
Since most people know these rules there are plenty of opportunities for you to disguise your true cards by drawing an amount inconsistent with the cards you have in your hand such as drawing only two cards when you only have a pair or one when you have three of a kind hiding your true hand quite effectively and adjust other peoples betting accordingly.