Recomendations to allocate TSP contributions

Your retirement is important and much like your career it comes down to you. My advice would be to not invest in anything you do not have a basic understanding of. In the case of the TSP look at the various funds, and their performance to date, also consider how much risk you are willing to take. Diversity is also important so you should spread it across at least a few funds.

It depends on what type of investor you are.  Active, semi-active or hands off.  I am a hands off investor so I like the L 20 options.  Since I plan to retire in 20 to 30 years I put it all in L 2040 and it will automatically allocate my funds more aggressive now and less aggressive as your retirement time gets closest. 

As a rule of thumb for contributing towards retirement investments, one needs to do the following:
1. Contribute up to the match for your TSP (if a civilian government employee) or your 401k or 403b (for non-profits)
2. Fully fund your IRA (Traditional or Roth IRA) up to $5,500 this year and for 2014 as well
3. Contribute up to the annual maximum of $17,500 (beyond the employer match) towards your TSP/401k/403b. Note that uniformed service members can contribute up to a maximum annual additional limit of $51,000 with tax-exempt contributions made to their TSP accounts while deployed to a combat zone
4. If you have more money to contribute outside your tax-advantaged accounts, then consider contributing to low cost well diversified mutual funds or ETFs (highly suggest Vanguard funds like Total Stock Market or Total International). Investment Costs/expense ratios matter!

With regards to your specific question about which TSP fund to invest in, as many have said, it all depends on your time horizon and your risk tolerance...I initially started out with the C Fund (Large Cap) then as I got close to retiring from active duty, I switched to Lifecycle 2030 to reduce some risks. Lifecycle funds rebalance between the C, S, F, I, and G Funds in relation to your time horizon. It all depends on how comfortable you are with managing your own asset allocation...if in doubt, I would suggest a Lifecycle fund.

The important part is that you started contributing to your TSP account (quite possibly the world's lowest cost retirement investment vehicle) and can harness the power of compounding. Sadly, as of June 2013, I read that only about 40% of active duty service members take advantage of the TSP.

For additional reading (in addition to the TSP website), I suggest the following:
http://www.bogleheads.org/wiki/Military_finances
http://themilitarywallet.com/category/tsp/