If a person feels he cannot concentrate on at least the first beracha of shemoneh essrei, the person should wait a while and daven later when he/she can. But a man should not miss zeman tefillah because of this. A woman who cannot concentrate, should not daven that tefilla.
If a person’s mind wandered throughout the first beracha, if the name of HaShem at the conclusion of the beracha was not yet said, the person should go back to Elokei Avrohom and repeat from there. If the name of HaShem at the conclusion of the beracha was said already, the beracha should be concluded, and the person should repeat the first beracha in their minds, not saying the words, but thinking their meaning. And the person should make sure to concentrate at the beracha of modim.
If the person concentrated for most of the beracha, but his/her mind wandered for a few words, the person should repeat from the place his/her mind wandered (if the name of HaShem at the conclusion of the beracha was not yet said).
The place we daven in–the Shul–has a special kedusha as a result of that function. HaShem said that a Bayis should be built to represent the place “where He is”(this is the Beis HaMikdash), and to serve Him there with the avodah he requests. Why HaShem davka wants a bayis, we do not fully understand clearly. But our Shuls are that Bayis!