This is a very broad subject and most of your questions I believe are answered in tzenes answer, I will try and give you some tips on macroing.
In every match, you always have different things to focus on. You have the following, but definitely not limited to, the following things to focus on: Minerals/vespine, Current Supply, Mini-map, battles, bases, armys, etc.
This is alot of things to worry about. The key is cycling through these things while your playing the game.
First off watch this video.
In my opinion Day is one of the best people to watch when trying to learn the game. Day  covers alot of what you need in that video.
Your specific issues
I keep on getting supply blocked
This is a problem everybody has occationaly. The key to staying on top of this is making a cycle: A mental checklist of things you need to go through in a loop. For example:Spawn Larve-->Check Supply-->Check Mini-map-->create drones/troops-->Spawn Larve-->...
This isn't necessarily my mental checklist but you get the point. When you do one thing, check the other. Practice is key, as in anything, to get good at something.
I'm using my larva injects fairly frequently, but whenever I get distracted I forget and my unit production suffers as a result
Again, Mental Checklist. The key is to not focus on the battle. Its hard to do especially when you want to micro your units, etc. One thing I do to keep myself from falling in that "watching" trap, is I reconfigured "Tab" to be my hotkey for switching around to my bases instead of "backspace". I then have all my queens hot-keyed to one control group and I Shift click spawn larve and tab around to all my bases and all my bases are instantly creating additional larve. No one can give you a easy way to remember to do all this. It is up to you to practice and get your mind-set to constantly scroll through your checklist.
I always end up with massive amounts of minerals and no gas
This is a complicated issue. This could be for a variety of reasons. But the normal solutions to it is this: Get more Gas Geysers to produce more gas, and use your minerals faster (Produce lings, create additional bases, etc)
I'm never sure when to expand
This is yet another tough answer. As a Zerg, you should try to keep +1 bases to that of your enemy.
Some basic tips include: Expand while attacking, when you have excess minerals, and when you need to gain a higher economic advantage, etc.
Other tips on macroing
Droneing, this is a HUGE issue many new players have. They focus on rushing and don't spend enough on economy. Basic Guidelines are if you rush, typically you are setting yourself behind your opponent in economy. This is true unless of course you do damage. I linked to Day 's videos a few times in this answer and here is another link that can help you in this respect. There are may other issues that I can't cover in one answer. But I suggest taking a look at Day's videos if you are looking to improve.