Hey, welcome to the game! Lizardmen are by far the best choice
Basics:
Assuming you haven't already, get the "Warhammer: Lizardmen" and "Warhammer Rulebook" - you can get a smaller (and cheaper) paperback version of the rulebook that covers all the rules. The larger one contains a lot of background and scenarios which, whilst interesting, are unnecessary.
Lizardmen Battalion, however if you want to avoid spending a lot then I'd start off with a focus on core and a champions:
Next purchase I would get:
- Skink Cohort (which you an use as either a Cohort or Skirmishers)
- Skink Priest
The third purchase I would focus on specials, which is where you could allow yourself more freedom.For me it'd be a fight between: Temple Guard to add a stronger fighting unit to whittle down your prey, Saurus Cavalry for a hard hitting offensive, Terradons (or Ripperdactyls, although I've never used them) for war machine/character hunting, or Salamanders to provide artillery support.
The first Rare unit you should be a Stegadon. Absolutely a Stegadon. You could use it as a Special, but the Rare unit version is better. I'd also focus on getting a couple of other specials first (as mentioned above).
Is converting easy? I would rather just get a old blood normally.
Kit-bashing is the easiest way to convert. Just stick a lot of different bits together to make your Saurus stand out and look how you want (I made a a trident for Skink Brave using three javelins, for example). The limit is your imagination, your spare bits, and your wallet.
Yes. They can be used as either a Cohort or Skirmishers.
Do you think i should go for a pack of skinks and a skink priest leading or stick with oldblood and saurus warriors first and get skinks after?
Saurus first to create a solid infantry core, and then branch out with Skinks for for ranged support.
Tehenhuain is mandatory when you're building an all skink army.
Just to clarify: mandatory as in he works wonders for a mono-Skink army, but he isn't mandatory rules-wise. My game group forbids special characters sadly.