Ok, I very much applaud your desire to learn more about the body. I will tell you this straight off: ignore, destroy, never listen to and ridicule ANY COACH who says "any weights will slow you down". I just wrote a novel length book underneath and I hope to hell you read it all otherwise I just really wasted a lot of time.
That is so completely false its equivalent would be me saying "the earth is flat and we never landed on the moon". We have abundant, complete, clear, and indisputable proof that is not true.
I get intensely angry just reading that!! You know what? People thought that was true for football players and wrestlers. What happened? They got stronger and faster when they lifted. They thought it was true for sprinters. What happened? The 100m world record is only, say 4 whole seconds FASTER now with sprinters who lift weights than it was before weights were involved in the sport decades ago.
I could go on and on. In fact, you want to know what happened to MMA athletes when they finally got serious about lifting? They got stronger, more powerful, and FASTER! The fact they can't box to save their life isn't the weights, it's the fact they don't train boxing adequately or have crappy "boxing" coaches along with poor technical efficiency. Their whole body explosiveness and power is far ahead of where they were before lifting seriously as part of their training.
OMFG I might have an aneurysm if I have to hear one more "trainer" say lifting makes a boxer slower. FUCK. God I could write a fucking BOOK on the reasons that's complete bullshit. Breathe....breathe....
Ok, soap box off.
1st) As a beginner to weight training the following is true: 1) any training now will make you stronger and probably more powerful. Speed is a function of punching technical efficiency with boxing, so it will not make you faster by default...but neither will it make you slower. Strength is the most essential as a complete newb to training--I am going to link a couple articles here which you need to read to understand the point--the point is very specific so do not take this as meaning you don't need to do speed work and power work (you do), and you don't need to condition (you really fucking do). Just the take home general point.... Link: http://www.T-Nation.com/free_online_article/most_recent/conditioning_is_a_sham&cr=
EDIT: Could only find the one. Maybe I can find the other later.
2) as a beginner, you will may possibly suffer bad DOMS soreness after lifting, which WILL impair your ability to be smooth and relaxing and "loose" while punching. This may or may not cause you to be slower in the very very short term--some people have trouble with it and others I have coaches never even get phased. It is only for a little bit: Rule of biology--every time you add something new into your program it increases DOMS, but as you get used to it it goes away and you can function just the same as before, if not better.
This happens with 100% of people that are new to weights or lifting--sometimes it even happens when you start push-ups if you've never done them before. DOMS will go away as you get used to training volume, a couple weeks max.
THIS CANNOT BE OVERSTATED--which is why I am putting it in caps--you need to recover from weight training appropriately. This will cut down on soreness and tightness, both good things for keeping your boxing training in high gear. Recovery from weight training is two part: 1) supplement with protein and carbs preferably during or after weight training 2) don't sit on your ass. Meaning, move around after your weight training and the day after as well. Soreness may or may not be unavoidable, but STIFFNESS is a function of time spent on your ass not moving blood through the body to clear metabolites and not stretching to keep range of motion.
Ever notice sometimes you feel more sore after taking a couple days off training than if you just did something light like jumping rope or jogging or push-ups? That's why. Light, very easy recovery work to move the blood around and specifically the muscles you trained with weights. Train legs? Move the legs through big ranges motion on easy shit. Train back? Stretch it and move it around to get the blood flowing with zero effort. Train arms or pushing movements? same thing, easy push-ups and stretches just to move around and keep cobwebs at bay. Not a workout--recovery and "loosening up". Big difference.
Those 2 things are key to adapting quickly and with minimal tightness and soreness to a new weights program.
Finally big point 3: weight training is not conditioning. Have a goal for your training time. Either condition or do strength and speed and power work. Not everything at once. You want to condition? Go condition. You want to lift weights for conditioning? Fine do it. But do not confuse your goals. Power and strength work with weights is 100% different than conditioning with weights. Same with speed work from conditioning work.
Case in point: Sprinters have learned this lesson well. Their max speed workouts are long rest periods, short runs working on efficient mechanics and maximum intensity (meaning trying to work up to their top speed), and/or acceleration. Their "conditioning" workouts on the other hand are brutal and geared around lots of volume with little rest, but done at a speed a little below their maximum--i.e. a "pace" they can maintain technically. They do not push all three at once in a training session.
Use that in your weight training. Either condition OR work power/speed/strength, not both, in one workout. A finisher for "conditioning" is fine, but do not make the workout an endurance fest. just the last 5 minutes of a workout, that's it.
More to come, if you read that novel.