Wow. For food words, consumables, and various trendy or pop culture items, I can see the propensity for adopting Western/English words written in katakana. But a table? That should be one of the older, more well-established words in any given language. So strange to see something so basic uprooted by a foreign loanword.
It's not the Kanji 一, it is to elongate the vowel of the character before. It is only used in Katakana. So テーブル, is pronounced like teeburu.
In hiragana, you just have to add an extra vowel, like おかあさん, would be okaasan. You must pronounce the vowel longer or you will sound like you are saying おかさん, or Mr. Oka. Hope this helps.
This isn't actually an "ichi". This small line is used in katakana and sometimes in hiragana to extend the vowel of the previous kana symbol, often to reproduce sounds from foreign languages like english:
カド ゲム: kado gemu カード ゲーム: kaado geemu (card game)
In this table case, the "e" sound from the "テ" is extended so it becomes a "tee", because this is actually the word "table", or "teeburu"
ー and 一 are almost the same looking thing
there is an app for iPhone/ipad called Learn Japanese!!. it has exercises to learn Hiragana and Katakana characters and once you have learned the characters there are lessons in basic Japanese phrases. It's a great help for learning, although it can't replace Duolingo and these discussion boards.
Check out JA Sensei. I relearned kana so many times over the years that it's not an issue with Duolingo, and the last way I practiced was through that app! Before that I just made my own flashcards from a Mario Paint instruction booklet and practiced writing the alphabets over and over (before the internet really took off lol). Either way works well depending on your learning style.