hey, welcome to OU. theres a lot of mons to use but generally, especially for new players, i recommend using mons that you feel comfortable using that are still strong. i recommend using some of the sample teams here and see what you enjoy and get used to the metagame, and then you can start building your own teams, choose a team that looks fun to you and any of the teams here are solid and viable. theres also some more in the OU room if you do the command !rfaq in there. Here is the link to the samples currently on the forum though:
https://www.smogon.com/forums/threads/sv-ou-sample-teams.3712513/
as far as mons to look out for, generally the viability rankings is a great resource:
https://www.smogon.com/forums/threads/sv-ou-post-home-viability-ranking-thread.3723863/ which basically assigns a rank to all the mons based on how well they perform their roles in the metagame. sometimes, depending on the team you use you'll need to look out for different mons, but generally when teambuilding I like to prepare answers for the major offensive threats like Kingambit, Baxcalibur, Valiant, Dragapult, and Enamorus and then make sure your team has answers to strong defensive mons like hazard setters, status spreaders, and other bulky mons from fulfilling their role or chipping your team down too much.
I'll also link a discord made by my guy
njnp which is dedicated to helping players improve at mons, specifically OU. theres also some resources in there pinned in #general-ou-discussion like teams and guides to help you play better.
https://discord.gg/P7ag6GHfuU