Sign up for contributions to TSP going forward.
Since you haven't contributed anything yet in 2016, you can still open and contribute to an IRA for 2016 (and possibly decrease your taxes) up to the tax filing deadline in April. Mock up your tax return several differ ways to choose the best option (Roth or traditional IRA) - there's two components to consider. If your AGI falls in range, you could be eligible for the Retirement Saver's credit of up to 50% of your contributions (up to $2k considered, but you can contribute to $5500) to either Roth or tIRA. Also, contributions to a tIRA reduce your AGI, and thus taxable income.
Roth has no tax deferral up front, but is tax free at withdrawal later.
Traditional has a tax deferral on contributions up front, but all withdrawals are taxed as ordinary income later.
So if there is little to no tax cost, prefer Roth. If tax avoidance is more important (especially at higher tax rates), prefer traditional. Since most people start off in low income (low tax brackets) and income rises later, the early years are often the best time to go Roth, and shift more to traditional later. Of course, it's not all or nothing - you can do proportions of each, too.