Low self-esteem is a huge issue for many of us. I agree that working out can do a lot to enhance that (as can several other things). As JohnSpotter said, it is good that you're starting now. Young people get better results faster. Even so, be aware that it is a 'long term' commitment (think 'years'), which is not a bad thing. In fact, it is a good thing. Just don't expect instant transformation. It might take two to three years to see dramatic results. The trick is just to keep at it and gradually work harder and harder.
I didn't start working out seriously until I was 63 years old! Worse, the gym I go to is on a college campus which means *most* (but thankfully not all) of the gym members are younger than you are. Worse yet, many of them are true jocks and have been training one way or another since high school. And at times the gym is extremely busy! Talk about intimidating!
However, I was and still am determined.
You've been given a couple good tips. Rene_Aensland's suggestion is a good one. I have a play list on my iPod of music that gets me moving and I keep it turned up. But there is way more to this than just that.
*I know what I'm doing*. I know when I walk into the gym what I'm going to do that day. I have a log sheet already prepared so I can keep track of how many sets and reps and what weights I used. I know which muscle groups I'm working that day. I've read and watched videos online so I know correct form for each of the exercises I'm doing. I know about keeping my attention focused in the muscle group I'm working. I know about agonists and antagonists, full range vs. partial range, the value of light weights and high reps and heavy weights and low reps. I know about rest periods in between sets and the advantage of varying them. I know about jacking myself up (with a pre-workout drink) and feeding my muscle and other tissues immediately after my workout. Etc., etc.
The point of all that is, for me all this is part of my focusing and giving myself confidence before I even set foot in the weight room.
Consequently I'm really not that worried what other people think.
Fact #1: I don't know what they think unless they tell me or show me in some way.
Fact #2: Most people only think about themselves, especially in the gym (which is a good thing).
Fact #3: I know more about what I'm doing than anyone else.
Fact #4: I'm not there to compete with anyone other than myself.
Fact #5: I'm not there to win anyone else's approval or praise.
I'm there to do my workout, period. True, people do look at one another. But why waist energy worrying about what they think or whether they are judging you? I see guys at the gym that I admire A LOT. To me they are inspiration, role models. Since most are way younger, there is a certain degree of envy on my part but I keep that in check. That is water under the bridge for me.
On the other hand, I also see guys who obviously don't know as much about form as I do. I see them doing things that I *know* can and probably will hurt them (I don't mean right then and there but over time). They've gotten away with it because they are young but one of these days their bad form, their ego or what ever, is going to catch up with them and they're going to be in a world of hurt.
I know better. Last thing I want at my age is a workout related injury! But, because I keep logs, I *know* that I'm improving. I can see and measure the results.
So, although I could be totally intimidated by going to my gym, it isn't like that for me at all. I love it. I look forward to it. I enjoy it. I do my workout and I even do more than my workout because I get pumped and start just doing stuff I enjoy for some extra volume.
Find what you love to do. Do more of that. Have fun. Don't worry to much about what others 'think'. Mostly they don't. ;)