It's no great revelation that the very act of stepping into the gym is one of the hardest parts. I can't tell you how many naps I've taken in my car at the parking lot of the gym.
It's like my mind throws a switch and makes me so sleepy that working out seems impossible.
But if you can just step inside you'll already feel better about yourself no matter what you do once there.
But what do we do once there?
Part of what I do as a trainer is write programming for people who train alone for various reasons. Many times they just don't know how to break apart the areas of the body and are over training some muscle groups while totally under training others.
So, your Step 1: Know what you're training before you go in.
This doesn't have to be a detailed play-by-play in the beginning. Try splitting the body into three movement patterns and do each one on a new day, rest, then repeat.
Day 1: Horizontal Pushing and Pulling Day 2: Lower Body Day 3: Vertical Pushing and Pulling
Step 2: Survey the land.
Use your warmup time, on a bike say, and check out the gym floor. What movements fit into the category you're training that day? Pressing a machine over head? That's a vertical push/pull day. Pressing a machine away at chest height, that's a horizontal push/pull day.
Pick three moves for legs, three for the push and the three for the pull. So your two upper body days will have six different movements.
Step 3: Have a plan on how the work should be done.
Shoot for around 3 sets of around 12 repetitions. Set the weight to something you feel you can do for maybe two more reps than where you're stopping, i.e. 12. Rest in between is about 45 seconds.
If a movement feels weird, stop. Ask for help, or use your phone and research some video descriptions.
Your goal is to be safe and to establish a routine. The habit of it all is critical so keep at it even when you aren't feeling it.
If this is overwhelming find a trainer or friend that can help.