Hello, I'm Szacho and I would like to help you a lot. I'm playing on Diamond with same nickname like here on forum, so you can add me and send me a message, if you will have any question or if you will need any help.
For now I will give you some advices here, so:
◘ Earning money on low levels
In my opinion the best way to do this is fishing, then catching pokes you killed (except noob Magikarp) and selling them to Santiago (Market <<<<< from Celadon PC for example). Usually it will be earnings, no loss but of course - not always. Remember to use only pokeballs or great balls - it seems to me that it pays the most.
Remember - fishing is only example, and there is a lot of pokes which can give you some easy cash from 'catch-sell-catch-sell-catch-sell' option. Like huberto said - also Mankeys belong to them.
Next thing is looting and selling plates. NPC/players can pay some nice cash for them and getting them is not extremely difficult, because each type of pokemon (first evolutions also) sometimes will leave them in a corpse.
Anyway the best option for making cash are quests, but about them at a separate point...
◘ Quests
There is a lot of quests on Pokemon Adventures. To make them, official PA fansites can be really helpful. Here is an example - official PAdventures polish fansite (with ENG language option also):
http://en.padventures.info/Missions
If you want to make any quest remember about checking what you really need (dacc, level, pokemon skills, items). You should also know, that in some quests you will need others.
◘ Which pokemons for new players
Like huberto said the best way for you is having some pokemons with different type. For example better option will be Charizard + Venusaur, than Charizard + Magmar.
I will show You my list of pokes I like from each type but you will need to choose, so:
Grass: Victreebel, Sceptile, Leafeon, Bellosom, Sunflora, Vileplume
Water: Blastoise, Vaporeon, Empoleon, Poliwrath, Kingler, Greninja
Fire: Magmortar, Flareon, Arcanine, Torkoal, Magcargo, Charizard
Ground: Donphan, Sandslash, Dugtrio
Rock: Sudowoodo, Golem, Rhyperior
Electric: Raichu, Electrode, Jolteon, Electivire, Zebstrika
Normal: Clefable, Stantler, Kangaskhan, Togekiss
Flying: Pidgeot, Fearow, Noctowl, Swellow
Psychic: Alakazam, Mr. Mime, Grumpig, Wobbuffet, Espeon
Dark: Mightyena, Weavile, Sableye, Hydreigon, Umbreon
* Remember - Most of them are dual-type, for example Golem (rock/ground) or Vileplume (grass/poison), so I ignored some types on this list (there is no Poison list for example)!
* Remember - If you will buy some pokemons, check which skills they have, because it's very important. You will need teleport and fly on dacc (ride and surf are optional and much easier to get). You can check everything in dex!
* Remember - You will need a healing pokemon (for example Clefairy/Clefable) - don't forget to buy or catch one!
* Remember - I wrote just
my propositions and I used their last form!
Okay... thats all for now, but remember - if you will need any help or you will have some questions 'catch' me on PA (Diamond - Szacho) and just PM me.
I wish you nice time here and in-game and of course
A LOOOOOOOT OF RAAAARE CATCHESSS!