You never deduct a contribution to a Roth IRA. That's the whole point.
If you are not eligible to make a Roth IRA contribution, you may be eligible to make a tax-deductible contribution to a traditional IRA. If you can't do that either, you can always make a non-deductible contribution to a traditional IRA. This would mean you can't deduct the contribution, your investment grows tax-free, but you pay tax when you withdraw. It's slightly better than putting the money in a regular investment account, but not much. It also causes paperwork problems when you withdraw from the traditional IRA.
A backdoor conversion is when you make a non-deductible contribution to a traditional IRA and then "convert" it to a Roth IRA. This is the only way to get money into a Roth account if you are not eligible to make Roth contributions. However, the rules are very complicated, and you really shouldn't even think about it unless you plan to convert all your funds in a traditional IRA at the same time, or you currently have no funds in a traditional IRA.
