To answer your first question, Wish is simply a two-turn move that involves recovering the HP of a Pokemon. When the user uses the move Wish, it will send it's wish up into the sky. At the end of the next turn, it will say "[Pokemon's] wish came true" and whoever is in battle on this turn will have half of the Wish user's maximum HP recovered for this Pokemon. Here are some things to know (As of Gen V):
1) You use wish. If you do not switch out your Pokemon on the next turn, the same Pokemon who used it will get the Wish (half of the Wish user's maximum HP restored)
2) You use wish. Then, on the next turn, you switch out your Pokemon. At the end of the turn, this new Pokemon will get the Wish
3) In doubles or triple battles, if you use Wish and you switch out next turn, the Pokemon who you bring in for that position will get the Wish
4) If by the next turn, your Pokemon in the position who used Wish faints, the Wish is not granted thus no Pokemon will get the Wish
5) Protect will not block Wish
6) If the opponent uses Snatch, they can steal the Wish
As for your second question, typically a Pokemon like Jirachi or Chansey would be a better Wish user since their most dominant stat by a large amount is HP - usually means they are ideal for status moves such as healing. Here, Jirachi has good HP and defenses whereas as Chansey has a lot better HP stat and at least one good defense, SpDef (though it's Def stat is the problem).
As for your choices, Alomomola has a great HP stat but it's defenses are weak. Togekiss has good defenses but mediocre HP stat. In terms of using the move Wish, both of them may work - the only difference, if you come down to a scenario where you have one Pokemon left, that is the one with Wish, it's a lot more ideal to have Togekiss since Alomomola's attacking and defense stats are around mediocre to poor. If you choose Alomomola, it will typically be your status-move user.
In general, I would personally choose Togekiss since it's base-stat total is higher and it's a lot more well-rounded in comparison to Togekiss. Furthermore, Togekiss' speed stat is a little higher than that of Alomomola, meaning that you have a slightly higher probability of using the move Wish before your Pokemon faints. So in general, I think you should invest more towards Togekiss (Alomomola isn't really the ideal option in comparison with all Pokemon for Wish). But do check out other Pokemon who can learn this move as well such as the two mentioned above.
Your Choice! Hope I helped! :)