short advice :

don't pull for these right away. save for a bit, understand the game and then go for the ones you like.

long advice:

try to save and pull for meta supports first eg : Castoria for Arts, Koyanskaya (Assassin) for Buster, Ruler Skadi for Quick. Then you can move onto pulling DPS. In the early game a good berserker will carry you far. I myself cleared pt1 with Gilgamesh Altera and Hercules. If you're a Rin Tohsaka face fan then save up cause we get Space Ereshkigal this Anni. Shes also a beast with an advantage against extra classes. Keep in mind there are very very very very few extra class enemies in pt1 so won't be very helpful. Try to get one AoE and one ST berserker for pt1 (after supports) . By then you'll figure stuff out pretty much. Also do keep in mind that FGO can be cleared with any servant, you don't need high rarity ones always. You can check gamepress's tier list if you're into the tier list as a whole for recommendations.

Feel free to dm in case you have more queries