Ah a topic near and dear to my heart... money.
First and most important piece of advice: don't listen to anyone else tell you "the best thing to do..." The only person in your exact life/financial situation is you, and no one else is able to consider all of those variables. You don't need to get a CFP license, but you should learn enough to at least have a basic understanding of what everything is.
Some quick points to start further research:
Roth/Traditional:
Roth
Contributions are post-tax (at your income tax rate)
Earnings are tax exempt
Traditional
Contributions are pre-tax (tax deferred until withdrawn, then taxed)
Earnings are tax deferred
Why you would use each
Roth: Pay taxes now (preferably in a low tax bracket), never pay again for any withdrawl
Traditional: Put off taxes now, and pay taxes later (hoping you're in a lower tax bracket later in life). Also, potential tax savings now as contributions are not taxed as income, potentially lowering your current tax obligation.
Both of these are available outside of TSP as well, with lower contribution limits. TSP limit is 18,500, outside is 5,500/yr for people under 50.
Sidenote: All agency matching contributions go to traditional, no matter what.
Funds
People like to throw the letters around without even knowing what they stand for. Learn about the indexes they track at the least, so you have an idea of what they are. You can google them. The TSP funds aren't unique, but the really great thing about them is the fees; they're nearly non-existent. Can't get that outside of TSP.
G: US Government bonds. People hate on this because its normally the "lowest" returns. But its for a reason: the TSP is given the ability to buy non-marketable US Govt T-bonds. Meaning they can't be sold. T-bonds are the baseline for any investment valuation, as they are basically considered "risk-free" (the gov't can't go bankrupt). They are non-marketable, so only held to maturity; there will be no loss on principle, ever. Cannot have negative returns.
F: Fixed income. Bonds from corporations. Not risk free. Higher yielding than T-bonds, because they are higher risk. The value can fluctuate (IE go down or up) from the principle investment. Can have negative returns.
C: Tracks the S&P500. Largest companies in the US. Turn on CNBC to see how its doing.
S: Small cap stocks. Tracks the Dow Jones U.S. Completion TSM. More volatile than large cap. Dependent on the health of the economy (Bad economy kills smaller businesses).
I: International stocks. MSCI EAFE. Companies outside the US, in the developed world (Europe, Asia, Australia)
Basic overview:
TSP: Fund Comparison Matrix
The L funds are simply a mix of the above funds. They automatically allocate based on the standard industry practices for your age (IE: 25 years from retirement, hold 80% stocks, 20% bonds). Funds like this are marketed as "set it and forget it." If you have no desire to ever think about your account, this would be the "recommended" way of investing it.
As I mentioned above, your entire financial situation needs to be considered. The TSP is only a piece of the whole, and you should at a minimum figure out where it fits so you can use its benefits and avoid its weaknesses. So many people throw out "max your TSP asap", and it's not always wise advice. The only universal advice I would give for anyone, would be to contribute the 5%. It's literally an extra 4% pay raise (they give you 1% no matter what).
My Federal Retirement - FERS / CSRS / Thrift Savings Plan