Nogzor'z
Smash Journeyman
For starters, try not to go overboard with learning all the tricks immediately (Parrying, rapid edge-cancelled eggs, Double jump cancel counter, etc.). While these things are good they require time to implement not only the timing but recognizing appropriate instances to utilize them.
Take the time to greatly familiarize yourself with yoshi. This relates to all characters really. Movement is rather important and knowing the limits of your character allows you to know what options are available at any given time.
If you can play against someone else, and, initially, get a feel for positioning and moving around while "under pressure" from an opponent. Being able to confidently move into and out of and position swiftly is pretty dang important. Understand specifics such as your general wavedash length, effective attack range (for all attacks), and jumping and aerial movement speed. Knowing these things will allow you to judge the correct spacing from an opponent to be considered safe for you, but dangerous for your opponent.
It can be easy for a yoshi player to be overwhelmed by an opponent since yoshi cannot jump out of shield, has issues with attack range, and some large lag time after some attacks. Take note when these disadvantageous situations occur and avoid them (getting stuck in shield being a major issue for new players).
Also, Understand how Yoshi's double jump (DJ) works as a recovery tool. When recovering against opponents, figure out the effective Damage limit that your double jump can absorb for all enemy attacks (it tends to vary a bit for stronger to weaker attacks). Use DJ at an appropriate height when returning to the stage so that both the option of land on the stage or grabbing the edge remain available. With that understand both
1. the Height of the DJ so that you can consistently "sweetspot" or grab the edge at the peak of the DJ, and
2. That the Dj can be cancelled at any point (preferably with Down-B or Up-B). Cancelling it with an attack will diminish upwards movement and allow grabbing of the edge when the full DJ will result in going too high.
End Note: Some of these specifics may be influenced by my opinions and there will probably be some second opinions/insights from other players.
As a rule of thumb, look through the yoshi guide thread here -->http://www.smashboards.com/showthread.php?t=53132 for general tactics, apply them and ask a lot of questions.
