I'm very weak. Where do I start with strength training?

I started with bodyweight, then machines, then dumbbells, then barbells.

We all start somewhere. Most people are going to suggest compound exercise, e.g. deadlifts, squats, etc. But if that's not your thing quite yet, you could always begin with bodyweight exercises and bands to ease into. Pull-ups, uprights rows, pushups, etc are compound moves too. I got back into the gym end of March and I didn't rush right back into barbells, instead I picked up dumbbells for most movements to just get my body back to the grind and to work out mechanics. That I am not recommending for you, but I think the principle could be applied. Either way, commit yourself to it and plan your gym time accordingly.

Yes, calisthenics are great! This is a good suggestion.

I started with bodyweight, then machines, then dumbbells, then barbells.

Me too! And I work at a desk also. Doing something > doing nothing. I still spend a lot of time looking at "how to" videos for the compound lifts. I'm progressing, but not quickly because I'm trying to make sure my form is good so if that means I stay at the same weight for a while, I don't care. The only person you need to compete with is yourself.

I can tell you that when I first went into the free weight section of the gym, I was intimidated and felt like they were all staring at me. Turns out, all the other people appear to be staring into space, trying to psyche themselves up for their next lift and I just happened to walk into their field of vision. But prepare yourself for that because that felt weird.

I don't think you'll be sorry that you are starting now. There is something very different about picking up more weight than you did last week. It's addicting.

I go to Planet Fitness and I use the strength training circuit of bicep and triceps curls, pull downs, chest press, fly, row, hip extension, etc. and go from one to the other, and adjust the weight (resistance) to what I can handle for that night. I recently started free weights, and look just like president Obama, lol.

Oh yeah, another thing that I did when I first started free weights was that I took body pump classes. It is a good way to get familiar with form and the movements with the free weights.

Don't be intimidated by the idea of working with a barbell. If the weight is too heavy, you can just lift with the bar. If you don't have access to a gym, you can use a broom handle or dowel to learn the motion.

A broom handle or dowel is a great suggestion. When I restarted after a bad ankle injury, I couldn't have done most of those exercises even with just a bar. I did a program (Strong Curves beginner) that started off with only bodyweight for at least the first month, and it was great for me. My form is way better than it was before the injury, and once I added weights I progressed faster because of all of that form work combined with gaining a baseline of strength.