I’m going to take a slightly different approach, although I generally agree with all of the advice here.

1. Establish an emergency fund. If you’ve been living paycheck to paycheck and do not have a significant amount of money in accessible savings, you’re taking a risk of not being able to handle something like a car repair or unemployment. Withdrawing funds from tax-advantaged retirement accounts can take time and incur significant financial penalties. The rule of thumb is to figure out what you spend in a month, and plan on an emergency fund that can carry you through 6 months of zero income. Some people do less, some do more, and if you’re really thinking about it you can figure out what expenses you can cut in order to make those savings go further.
2. Putting money into a matched 401k is a no-brainer, and going with an index fund or retirement date fund is the easiest way to go. However, realistically examine the expected savings by the time you plan to retire. This tells you how much you’ll be able to draw down and for how long. I’m going off of memory here, but I think the consensus safe draw down rate is 4% per year. That means $1M in retirement savings will give you about $40k per year to live on (not including things like social security). Depending on where and how you live, this might be sufficient. You’d have to plan for it though, which is my point.
3. There are plenty of retirement calculators online to help with this. You enter your age, when you want to retire, the amount you’re saving, and it will tell you what your savings will be when you’re 65 (or whatever) and how long it will last at different draw down rates. Some will let you estimate things like rate of return too. Be realistic.
4. Realize that the closer you get to retirement, the more conservative your investments should be. To paint with a very broad brush, low risk=low reward, high risk=high reward. The further you are from retirement, the longer you have to recover from a downturn. Look at the retirement date targeted funds - they move over time from a more speculative set of investments to a more reliable one. What I’m saying here is that you’ll read things about being able to plan around a 10% rate of return. That’s the average for a stock based portfolio, and it can swing around quite a bit. Individual stocks have a higher risk than an s&p index fund, and the index fund will have a higher risk than a conservative, income-oriented fund. Remember that when you’re using those retirement planning web sites.