Welcome to the club.

It is early January and (in parts of the country not frozen completely solid) the game is afoot! The game of: "Who will still be in the gym come Valentines?" There are a goodly number of folks who have resolved to... something in the gym. To you I say, Welcome!
First off, you are here in the gym. This means that you have joined an already exclusive club of "folks who exercise." You have also joined a sub-sect called "folks who go to the gym." Eventually (hopefully) you'll join the exclusive (and ever shifting) club of "folks who are in shape."  Due to an injury and 2.5 weeks of sloth and gluttony, I personally am not currently a member of this group, but membership isn't difficult it just takes persistence and effort.
Some basic advice for getting into this exclusive club and staying there (injuries aside).

1.) Have some fun. Seriously. there should be an element of play in your exercise, even if you aren't playing a game. You should get some joy out of it.

2.) It's not all going to be play. There are a lot of parts to this thing we call fitness. It's not really a thing so much as a series of achievements that have to be unlocked.. and all have to remain at a certain level at any given time for one to be considered 'fit.' I don't know what you don't like. I personally don't like mobility work and aerobic conditioning. These parts of my fitness are the ones I always manage to "forget." Temptation is strong, but don't fall into that trap. (strength, aerobic fitness, anaerobic fitness, mobility, body control, and posture are the general categories I use to evaluate folks) Make sure you're working on all of these qualities. Make sure that if you are seriously deficient in one you study it, and put focused effort towards it. Do your homework.

3.) Don't judge other folks on what they are doing. It's pretty easy to feel damn superior at the gym (well anywhere folks are putting their priorities out in plain view). You don't know anything about the person you're looking at, you don't know what their priorities are, and may not know their injury history. Right now, I have an injured leg. Doesn't effect my gait at all. I look perfectly healthy but I can't do any lower body work at all. It would be easy for someone to watch me train and slag me off for not "lifting legs." connected to this...

4.) Be positive. Look around and look at the positive stuff folks are doing around you. I used to be the aggro Henry Rollins aggro minded (young) dude who hated everyone, snarled with derision at everyone and everything around me.
That guy? "HA, no legs!"
This girl? "the ELIPTICAL, WHAT ARE YOU A HAMSTER!!?"
Well what about this dude, he's pretty big? "He can't even do a single pull-up, and call me when you're going to do the other half of those reps on the bench broheim!"
But now, I've been in the game too long to deride anyone. I've been in situations, or seen people for whom just about everything is appropriate, and for the few situations where I do know the story and I do know that people are doing things that are silly/potentially injurious I feel sorry for the pain that's going to inflict upon them. They're going to break, and it's going to hurt.. possibly forever. That's a sad thing. Ok.. back to you.

5.) Lift some weights. Weights that weigh a challenging amount. That will develop these things called "muscles." You might have seen these on television and at the movies. They go a long way in getting you to that "In shape people" club.

6.) Eat food. Here's what I want you to do. I want you to go to the store, and buy only food that has no brand name. So if the sign says "apples" good to go. "beef" similarly. "Kellogg's PostNabisKraft toasty 102 calorie puffy packs" NOPE!

Now I know Dole sells pineapples and Chiquita sells bananas, but (in spite of their best efforts) you don't go to the store looking for Dole bananas, you go for bananas and if they have this brand or that.. doesn't matter. Bananas are food, 'toasty 102 calorie puffy packs' are not food. They are food based ingredients processed together to generate a hyper-palatable food product. These products are designed with consumption in mind, not health. Putting hyper-palatable food in small bags just means you're going to produce a lot of trash. It's good marketing, and bad nutrition, don't fall for it.

7.) Do something that makes you breathe hard. Preferably some short stuff that makes you breathe really hard for a while, and some longer stuff that makes you breathe a little hard. Mix it up. Move in different ways, and incorporate play as much as you can. If you have kids, race them the length of the block. Put them on their bikes and run beside them, and make them tell you stories. Exercise is a lifestyle, and if you model that you'll build people who take it for granted.

8.) Roll, Crawl, and move from the floor to standing. I learned a ton from Charlie Weingroff but this was cemented in my time this summer/fall at Seattle Dojo Judo practice. Learning to tumble, to move on the floor, to move in a quadruped motion, and to go fluidly from prone to standing is one of the best ways to improve general coordination. Combine that with the single most prevalent killer of people over the age of 65 is 'injury due to accidental fall' then it stands to reason practicing falling without getting injured is kind of important.

9.) Take the time to develop the habit. Keep to a hard schedule for 6 weeks. No reschedules, no skipped sessions. Go, and exercise as planned for 42 days. Take scheduled days off, but keep to your plan, and you will find it much easier to stay on target the rest of the year.

10.) Plan to do less than you think you need to, and give yourself room to overachieve. This is a tough one, and hard to explain, but think of it this way. A few years ago a good friend of mine was doing a stairclimb. It's sort of like a 5k, up a building staircase. He was well prepared, but didn't have access to a tall building to practice on. My pre-race day advice to him was:
 "Start slow. Slower than you think..no even slower than that."
Fatigue sets in and you end up well past your capacity. You can always pick up the pace the last 1/3 if you have extra gas in the tank, but once you're gassed out.. you're done. Race day came and went, and his response:
"I started too fast."
If you are not training for a competition in the near future, and you're just trying to get into shape, have a baseline plan, and give yourself bonus work that you can add if you're feeling baddass. Don't start too fast and then blow up the whole plan.

Mahalo.