Welcome aboard, and thanks for your service. The most important thing you can do now, at Age 24, is to begin regularly contributing as much as you are comfortable investing. Every time you get a promotion or a time in service increase, or a January pay raise, take at least a part of that and make it an increase in your contribution. There is no substitute for the power of "time in the market". The longer you can keep your money in, the longer it has to work for you. You can think about HOW you want to invest it later- whether in one of the L funds, or actively managing it yourself- either way, the more you are able to comfortably add to your contributions, the better off your retirement will be. By adding a little at each pay increase, you'll hardly even miss the money. Add in those opportunities to kick in higher amounts when you are deployed, or get bonuses, and you'll soon have a nice chunk of change to work with.
Good luck, and welcome.