Building a gym workout plan is really no different than building any other workout plan. In reality they are both the same thing because if you subscribe to the advice on this site then you really don't have much choice but to focus on training in a gym. Lifting heavy weights requires space and lots of weights. Even though I workout every day, maintaining a home gym requires too much space and too much money.
Now if you are like most people and paying for a gym membership (mine is free) then you might be able to look at the cost of your membership over a period of time and justify the expense of turning your garage into your own home gym. If you have somebody to workout with you at home to provide the safety of a spotter then a home gym might be the way to go.
Regardless of whether you gym is in your house or at some other facility building a gym workout plan is pretty simple.
Determine when you are going to visit your gym and perform your workout program. Having a daily time is good so that it can become part of your routine and you don't have to drag up the motivation to actually go. There have been numerous times that I hopped in my car and drove to the gym only to realize that it was my scheduled off day. Humans are creatures of habit and once you build the habit of daily workouts you will find it happens without any motivation required.
Survey the equipment at your gym and determine if you are going to have any problems with one gym workout plan versus another. For example if there is only one squat rack in your gym and it is constantly loaded with people then you might want to come during off hours of be the first one in the gym during lunch time. You might have a gym that doesn't even have any free weights and so you are eyeballing the machine to see which exercises are interchangeable.
Machines are not interchangeable with free weights. If your gym doesn't have free weights then you need to find another gym or build your own home gym. Machine training is just not the same.
Determine your workout routine. I've already covered a beginner gym workout plan (http://www.liftingheavy.com/free-workout-plan.html) with exercises, sets, and reps for strength training so I won't beat that dead horse.
Execute your plan. This can be the most difficult part for those just starting out so I want to spend a little time on it. Executing is what people struggle with the most. The cycle is pretty predictable where someone decides to lose weight, build muscle, get in shape, or whatever. They start out really motivated by buying books, equipment, programs and anything else they feel is necessary before starting a gym workout plan and decide to "start on Monday".
From there it goes one of three ways:
The bottom line here is that you have to stick with it. Don't let one bad meal or one missed workout turn into a bad day or a bad week. If you miss a workout, stick to the schedule. Two workouts in a week are better than one. If you screw up a meal and eat a pile of junk then just let it go and move on. Stick to the plan and get back on track with the very next scheduled meal. Don't say well I had a donut for breakfast so might as well eat this pizza for lunch. Same rule applies, two healthy meals a day are better than only one.
So in summary for your gym workout plan you need to pick your gym, know what you are going to do, know when you are going to do it, and do it. It really is as simple as that. If you are in doubt go with the strength training program. Strength training is beneficial in numerous ways and it really is the best way to get started and see results quickly.