Who to make your buddy..

If you want to get into gyms, you need the highest CP pokemon you can get, and this means Tyranitar or Dragonite. Larvitars are more common than Dratinis, so every time you see a Larvitar capture it with a Pinup berry to get max candy. Also make Larvitar your buddy to evolve a Tyranitar ASAP.

The fastest way to power is Eevee. All Eeveelutions are among the strongest of their type. Eevees are quite common in most regions and you only need 25 for an evolution.

If your biome is desert ryhdon ( evolve )would the next best thing after the usuall snorlax , dragonite and such , also geodude ( 1 km in buddy ) and should have easy access to it

I'm level 30 and my advice to my litter brother who started a month ago is to walk with snorlax or chancey if you have one of them ( he have chancey ), if not then the rarest of magicarbs , dratinis ,larvitars .
Any thing else he will get it in no time and gyms will be conquered only by leveling up

Depends a little bit if your team is dominant or not. if they are you need high CP as possible.
Eeveelutions are very powerfull and easy to access.
And Rhydon (def)/Golem (att) should be easy accessible in a desert area.

For attacking, Arcanine, Exeggutor, Rhydon, and Eeveolutions are your pals. The most important thing to consider when attacking a gym is getting as much advantage as you can get by choosing the matchups.

As a rule, getting specialized attackers is a good practice, know the most common matchups:

Blissey: Machamp, Exeggutor, Flareon are good choices
Snorlax: Same as above
Dragonite: Piloswine, Cloyster and Dewong (2x SE). Jolteon performs ok vs dragonite (neutral damage)
Gyarados: Jolteon... or jolteon... maybe magnemite or voltorb with 2 electric attacks
Rhydon: Vaporeon, venusaur, exeggutor with grass type attacks (BS/SoB), even machamp performs well.
Vaporeon: Exeggutor, Venusaur, meganium, even Paras with Solar beam can make short work of them.

As for defending, if you can't afford actual good defenders (Blissey/Snorlax), Rhydon and vaporeon do get a good placement and are generally easy to get and power up.