I love cardio but am starting to do weights and don't want to overdo it. Advice?
I have lots but we’ll start with form over everything. It is important to perform the movement correctly and train that. This will provide the most benefit as the weight goes up.
When you first start a thing it will make you sore. If you keep doing said thing the soreness will abate generally within the first 2 weeks assuming you’re not crushing heavy weights and you are otherwise hydrated and have your diet dialed in to feed muscles. Dont let soreness slow you down But also don‘t rely on it as indicator of successful production, either.
I don’t know what flavor of poison your cardio is but you probably want to pick movements that potentiate as well as antagonize that thing. For example, let’s say running is your jam. Potentiators are things like squats, lunges, and sled pushes. Antagonists might be things like hanging leg raises, abductor/adductor (good girl bad girl) machine, and hamstring curls or glute ham raises. Obviously this is focusing on lower body and maybe you are looking to get some action outside your cardio focus like upper body for runners. In this case the same idea applies. If you are working chest then also do some band pull aparts and lat pull downs to antagonize (aka balance) the movements.
Free weight movements are long term more effective and safer to perform. Do these where possible. Machines offer a false sense of safety in that you can’t perform it wrong per se and you can’t really drop it but they actually force you into movement patterns that are completely linear and unnatural as well as encourage imbalances. Machines have a place and can be used just fine but should not be your mainstay for long term effective resistance training if you have free weights as an option. Provided a linear machine and a cable operated machine for the same general movement choose the cable.
If you want to progress the weights must go up. Find a rep range you want to work. Let’s say 8. Get through your initial soreness period and then start tacking on weight in manageable increments until 8 is not possible. Back down the weight until 8 is just possible. Do that for a while until 8 is no longer a struggle to finish. Increase the weight so that 8 is again a struggle to finish. Rinse and repeat ad nauseum or until desired results are achieved. That’s the basic idea behind what progressive overload weight training is about.
For maximum effect do not do the same things every single time forever and ever amen. Let’s say you want to grow your triceps. You may start with cable push downs. Sometime switch to skull crushers. Another time do over the head cable presses. There are a million variations but the general idea is to A) approach the movement from various planes and B) not succumb to an efficiency pattern that will inevitably result in diminishing returns. In these cases, with a trained muscle group, soreness can be an indicator of productivity in the sense that you have definitely shaken things up.
A lot of the above requires passing knowledge of how to do a thing or what things do what to which body parts. All of this comes with time and experience. Fraternize and socialize in the gym. Ask questions about what people are doing and why. The more successful a person looks at doing a thing the more likely they are to have done all the wrong things and have some decent advice. Advice should be accepted when requested but, in general, rejected if offered unsolicited. The exception to that rule is if the person expresses concern for your safety. Like, if they say doing this thing that way can hurt you then probably listen to them It still get second and third opinions while you stop doing the thing that way in the meantime. For example, some people will tell you deadlifts and squats will harm your back and they are wrong. However, doing deadlifts and squats the wrong way is a great way to wreck a back (Remember train the form, the weights will come). Initially you may want to just google exercises according to the target muscle groups. Internet videos are generally pretty good about depicting correct form.
That was my highly abridged, edited, and pauciloquent answer to your question. If you want more be specific because that was a very wide open question you asked.