Summoner is a late-game class; you're gonna be meh at best in pre-hardmode and only alright until post-Plantera (assuming she drops the Pygmy Staff first try). Using other weapons to supplement your damage is a must, but that can be whatever you want from whatever class really, unless you're using some mana-guzzling weapon like Last Prism. In pre-hardmode, pretend you're not playing summoner at all until you've beaten Queen Bee a few times and can craft bee armor and perhaps the matching staff. From there, get your Imp Staff if you haven't already (you can actually get this before fighting Queen Bee), go into hardmode and farm up 78 spider fangs, and keep your eyes peeled for the following milestones in power:
The Optic Staff which you can craft from Souls of Sight (The Twins). This can in theory be skipped if you're having trouble getting a black lens, but if you can make it I would heavily advise it.
The Pygmy Staff dropped by Plantera. Having this is when you well and truly go live, since carrying it causes the Witch Doctor to sell Pygmy Armor (inferior to Spooky Armor but oh well) and matching necklace as well as the Hercules Beetle (you'll want one of these later).
Pumpkin Moon event just in general. From Mourning Wood's drops, Spooky Wood is used to craft your armor, and the Necromantic Scroll increases your minion slots (you'll want two of the latter, one of which combines with the aforementioned beetle). Pumpking also drops a Raven Staff which is pretty good.
Stardust Fragments to create your endgame weapons, and Moon Lord for your endgame armor/sentries.
Martian Saucers are also good for you since they drop the Xeno Staff (poor man's Stardust Cell Staff) and the UFO mount, but both of these drops are so unreliable on such a rare event that I wouldn't blame you for thinking "can't I just skip that?"
In combat, you are the squishiest class in the game. Melee class armor will have higher armor than your post-Moon Lord armor as early as gathering hardmode ores, with the rest of the classes matching or exceeding your 38 endgame armor as "early" as chlorophyte. So rather than investing your reforges into Warding, I find it better to go full damage with Menacing everything (the best modifier for summon weapons is Ruthless, by the by) and mastering the art of dodging. The UFO mount, Rod of Discord, or even a good set of wings (Betsy?) will do you wonders in this respect. If you want to be a tanky summoner on the other hand, invest in the Valhalla Knight set available from the post-Golem Tavernkeep. Its 64 armor is higher than what rangers get post-Moon Lord, and the health regen is nothing to scoff at either. Keep in mind that by doing this, you'll need to get a UFO mount or other flying mount with sufficient airtime, and sacrifice your wings and every other mobility accessory for pure summoning power. I'd suggest doing this on a normal summoner too, but I'd super-suggest it on Valhalla Knight summoner.
Speaking of the Tavernkeep, I hope you like Old One's Army. Because you'll be doing that event a lot as a summoner. There's no summoner gear directly related to the event, but the sentries available for Defender Medals are usually quite powerful for where you are in the game, so grinding out those 100 medals for the Ballista Staff is a good investment. You'll be doing even more grinding if you want to go Valhalla Knight as well. On that note, take a protip from me, and upgrade your ballista ASAP each time the Old One's Army increases in tier (after killing a mech and post-Golem). Getting the upgrades is that much harder if you're starting with the small baby ballista against a tier 3 army.
I probably missed something but I only got out of bed a little bit ago and felt like offering up as much advice as I could on this topic. If I did, let me know and I'll get back to you on it. I should probably also clarify that I haven't owned Terraria as long as most players, but as if to compensate I've exclusively played summoner in all my playthroughs (except for one unfinished Calamity run where I tried the mod's rogue class). Good luck, aspiring new summoner.
