If you are baptized, confirmed and have received first Holy Communion
OK, I know nerves make this more difficult than it sounds but it really is as simple as:
Find mass times
Walk into Church shortly before a Mass is due to start.
Do not receive Holy Communion
Take a bulletin
Nobody is going to know you haven’t been in a while or scold you over it (at least they shouldn’t). You may not be familiar with the responses but you can listen at first. If you’d like to know them you can get a missal or an app and you can join in. A missal at least would require some knowledge first and I wouldn’t recommend buying one unless you’re confident that you will return.
The bulletin should have information about things you can get involved with. If not hopefully the parish will have fellowship after Mass during which you can talk to the other parishioners and get information about other things going on in the parish.
If you decide to keep up then one more important step
Go to Confession.
After that step is complete you can receive Holy Communion. Again, I’d make sure that I was going to stick with it before that.
If you have not received the sacraments of Baptism, Confirmation and Holy Communion or are missing any one of them then you can still go to Mass and get involved with the Parish, but you will need to ask the Priest about RCIA. He should be able to point you in the right direction. RCIA is instruction in the faith for adult converts or children above the age of reason. At the end of it if you wish you will receive whatever the sacraments you are missing.